Employee Benefit Plans (Details 5) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Details of investment out of defined benefit plan assets
Defined benefit plan assets	$ 443,962	$ 404,335	$ 427,131	$ 443,962
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	443,962	404,335
Actual return on plan assets	(593)	43,662
Value of plan assets, December 31	427,131	443,962
Limited partnership interests | Significant unobservable inputs (Level 3)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,114	7,557	$ 7,654	$ 8,114
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	8,114	7,557
Actual return on plan assets	(655)	510
Capital contributions to limited partnership interest	1,319	656
Capital distributions from limited partnership interest	(1,124)	(609)
Value of plan assets, December 31	7,654	8,114
Defined Benefit Pension Plan
Details of investment out of defined benefit plan assets
Assets (as a percent)			100.00%	100.00%
Defined benefit plan assets	443,962	404,335	$ 427,131	$ 443,962
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	443,962	404,335
Actual return on plan assets	(593)	43,662
Value of plan assets, December 31	$ 427,131	443,962
Ranges of targets for the allocation of invested defined benefit plan assets
Targets for the allocation of invested defined benefit plan assets, minimum (as a percent)	100.00%
Targets for the allocation of invested defined benefit plan assets, maximum (as a percent)	100.00%
Defined Benefit Pension Plan | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	$ 443,962	443,962	427,131	443,962
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	443,962
Value of plan assets, December 31	427,131	443,962
Defined Benefit Pension Plan | Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	2,841	2,841	2,764	2,841
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	2,841
Value of plan assets, December 31	2,764	2,841
Defined Benefit Pension Plan | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	433,007	433,007	416,713	433,007
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	433,007
Value of plan assets, December 31	416,713	433,007
Defined Benefit Pension Plan | Recurring basis | Significant unobservable inputs (Level 3)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,114	8,114	$ 7,654	$ 8,114
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	8,114
Value of plan assets, December 31	$ 7,654	8,114
Defined Benefit Pension Plan | Equity securities
Details of investment out of defined benefit plan assets
Assets (as a percent)			67.00%	65.00%
Ranges of targets for the allocation of invested defined benefit plan assets
Targets for the allocation of invested defined benefit plan assets, minimum (as a percent)	30.00%
Targets for the allocation of invested defined benefit plan assets, maximum (as a percent)	30.00%
Defined Benefit Pension Plan | Debt securities
Details of investment out of defined benefit plan assets
Assets (as a percent)			31.00%	33.00%
Ranges of targets for the allocation of invested defined benefit plan assets
Targets for the allocation of invested defined benefit plan assets, minimum (as a percent)	52.00%
Targets for the allocation of invested defined benefit plan assets, maximum (as a percent)	52.00%
Defined Benefit Pension Plan | Other
Details of investment out of defined benefit plan assets
Assets (as a percent)			2.00%	2.00%
Ranges of targets for the allocation of invested defined benefit plan assets
Targets for the allocation of invested defined benefit plan assets, minimum (as a percent)	18.00%
Targets for the allocation of invested defined benefit plan assets, maximum (as a percent)	18.00%
Defined Benefit Pension Plan | Common collective trust funds: | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	$ 286,244	286,244	$ 286,000	$ 286,244
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	286,244
Value of plan assets, December 31	286,000	286,244
Defined Benefit Pension Plan | Common collective trust funds: | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	286,244	286,244	286,000	286,244
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	286,244
Value of plan assets, December 31	286,000	286,244
Defined Benefit Pension Plan | Equity index funds | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	93,415	93,415	94,751	93,415
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	93,415
Value of plan assets, December 31	94,751	93,415
Defined Benefit Pension Plan | Equity index funds | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	93,415	93,415	94,751	93,415
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	93,415
Value of plan assets, December 31	94,751	93,415
Defined Benefit Pension Plan | Midcap index funds | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	90,159	90,159	88,267	90,159
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	90,159
Value of plan assets, December 31	88,267	90,159
Defined Benefit Pension Plan | Midcap index funds | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	90,159	90,159	88,267	90,159
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	90,159
Value of plan assets, December 31	88,267	90,159
Defined Benefit Pension Plan | World equity index funds | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,759	8,759	8,511	8,759
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	8,759
Value of plan assets, December 31	8,511	8,759
Defined Benefit Pension Plan | World equity index funds | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,759	8,759	8,511	8,759
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	8,759
Value of plan assets, December 31	8,511	8,759
Defined Benefit Pension Plan | U.S. equity market funds | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	93,911	93,911	94,471	93,911
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	93,911
Value of plan assets, December 31	94,471	93,911
Defined Benefit Pension Plan | U.S. equity market funds | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	93,911	93,911	94,471	93,911
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	93,911
Value of plan assets, December 31	94,471	93,911
Defined Benefit Pension Plan | Fixed maturities | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	146,763	146,763	130,713	146,763
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	146,763
Value of plan assets, December 31	130,713	146,763
Defined Benefit Pension Plan | Fixed maturities | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	146,763	146,763	130,713	146,763
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	146,763
Value of plan assets, December 31	130,713	146,763
Defined Benefit Pension Plan | U.S. government direct obligations and U.S. agencies | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	9,308	9,308	6,753	9,308
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	9,308
Value of plan assets, December 31	6,753	9,308
Defined Benefit Pension Plan | U.S. government direct obligations and U.S. agencies | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	9,308	9,308	6,753	9,308
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	9,308
Value of plan assets, December 31	6,753	9,308
Defined Benefit Pension Plan | Obligations of U.S. states and their subdivisions | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	18,838	18,838	19,074	18,838
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	18,838
Value of plan assets, December 31	19,074	18,838
Defined Benefit Pension Plan | Obligations of U.S. states and their subdivisions | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	18,838	18,838	19,074	18,838
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	18,838
Value of plan assets, December 31	19,074	18,838
Defined Benefit Pension Plan | Corporate debt securities | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	107,125	107,125	93,811	107,125
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	107,125
Value of plan assets, December 31	93,811	107,125
Defined Benefit Pension Plan | Corporate debt securities | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	107,125	107,125	93,811	107,125
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	107,125
Value of plan assets, December 31	93,811	107,125
Defined Benefit Pension Plan | Asset-backed securities | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,444	8,444	8,149	8,444
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	8,444
Value of plan assets, December 31	8,149	8,444
Defined Benefit Pension Plan | Asset-backed securities | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,444	8,444	8,149	8,444
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	8,444
Value of plan assets, December 31	8,149	8,444
Defined Benefit Pension Plan | Commercial mortgage-backed securities | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	3,048	3,048	2,926	3,048
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	3,048
Value of plan assets, December 31	2,926	3,048
Defined Benefit Pension Plan | Commercial mortgage-backed securities | Recurring basis | Significant other observable inputs (Level 2)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	3,048	3,048	2,926	3,048
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	3,048
Value of plan assets, December 31	2,926	3,048
Defined Benefit Pension Plan | Preferred stock | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	310	310	280	310
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	310
Value of plan assets, December 31	280	310
Defined Benefit Pension Plan | Preferred stock | Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	310	310	280	310
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	310
Value of plan assets, December 31	280	310
Defined Benefit Pension Plan | Limited partnership interests | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,114	8,114	7,654	8,114
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	8,114
Value of plan assets, December 31	7,654	8,114
Defined Benefit Pension Plan | Limited partnership interests | Recurring basis | Significant unobservable inputs (Level 3)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,114	8,114	7,654	8,114
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	8,114
Value of plan assets, December 31	7,654	8,114
Defined Benefit Pension Plan | Money market funds | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	2,531	2,531	2,484	2,531
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	2,531
Value of plan assets, December 31	2,484	2,531
Defined Benefit Pension Plan | Money market funds | Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Details of investment out of defined benefit plan assets
Defined benefit plan assets	2,531	2,531	$ 2,484	$ 2,531
Additional information about assets of the Defined Benefit Retirement Plan measured at fair value on a recurring basis and for which the company has utilized Level 3 inputs to determine fair value
Value of plan assets, January 1	2,531
Value of plan assets, December 31	2,484	$ 2,531
Post-Retirement Medical Plan
Expected payments
Expected employer contribution to the plan for the next fiscal year	787
Supplemental Executive Retirement Plan
Expected payments
Expected employer contribution to the plan for the next fiscal year	$ 3,337