Employee Benefit Plans (Details 5) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Details of investment out of defined benefit plan assets
Defined benefit plan assets	$ 443,962	$ 404,335
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	404,335	336,534
Actual return on plan assets	43,662	62,701
Value of plan assets, December 31	443,962	404,335
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
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	7,557	6,485
Actual return on plan assets	510	853
Purchases	656	630
Issuances	(609)	(411)
Value of plan assets, December 31	8,114	7,557
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
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	404,335	336,534
Actual return on plan assets	43,662	62,701
Value of plan assets, December 31	443,962	404,335
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	404,335
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, December 31	443,962	404,335
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	3,799
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, December 31	2,841	3,799
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	392,979
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, December 31	433,007	392,979
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	7,557
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, December 31	8,114	7,557
Defined Benefit Pension Plan | Equity securities
Details of investment out of defined benefit plan assets
Assets (as a percent)	65.00%	63.00%
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)	33.00%	34.00%
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%	3.00%
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	256,509
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, December 31	286,244	256,509
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	256,509
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, December 31	286,244	256,509
Defined Benefit Pension Plan | Equity index funds | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	93,415	82,440
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, December 31	93,415	82,440
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	82,440
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, December 31	93,415	82,440
Defined Benefit Pension Plan | Midcap index funds | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	90,159	82,674
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, December 31	90,159	82,674
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	82,674
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, December 31	90,159	82,674
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	8,186
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, December 31	8,759	8,186
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	8,186
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, December 31	8,759	8,186
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	83,209
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, December 31	93,911	83,209
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	83,209
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, December 31	93,911	83,209
Defined Benefit Pension Plan | Fixed maturities | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	146,763	136,470
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, December 31	146,763	136,470
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	136,470
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, December 31	146,763	136,470
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	19,088
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, December 31	9,308	19,088
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	19,088
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, December 31	9,308	19,088
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	14,973
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, December 31	18,838	14,973
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	14,973
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, December 31	18,838	14,973
Defined Benefit Pension Plan | Corporate debt securities | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	107,125	91,860
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, December 31	107,125	91,860
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	91,860
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, December 31	107,125	91,860
Defined Benefit Pension Plan | Asset-backed securities | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,444	7,902
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, December 31	8,444	7,902
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	7,902
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, December 31	8,444	7,902
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	2,647
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, December 31	3,048	2,647
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	2,647
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, December 31	3,048	2,647
Defined Benefit Pension Plan | Preferred stock | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	310	700
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, December 31	310	700
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	700
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, December 31	310	700
Defined Benefit Pension Plan | Limited partnership interests | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	8,114	7,557
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, December 31	8,114	7,557
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	7,557
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, December 31	8,114	7,557
Defined Benefit Pension Plan | Money market funds | Recurring basis
Details of investment out of defined benefit plan assets
Defined benefit plan assets	2,531	3,099
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, December 31	2,531	3,099
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	3,099
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, December 31	2,531	3,099
Post-Retirement Medical Plan
Expected payments
Expected employer contribution to the plan for the next fiscal year	606
Supplemental Executive Retirement Plan
Expected payments
Expected employer contribution to the plan for the next fiscal year	$ 4,874