Employee Benefits (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest percentage added to the five-year trailing average of five-year Treasury Securities	1.50%
Interest percentage accrued on employee account balances, Minimum	3.00%
Interest percentage accrued on employee account balances, Maximum	5.00%
Interest percentage accrued on employee account balances, Lower end of range for current year	3.00%
Interest percentage accrued on employee accounts balances, Upper end of range for current year	3.01%
Thrift Plan - Defined Contribution [Abstract]
Employee base compensation company match employee contributions (in hundredths)	6.00%
Employer matching rate for employee contributions minimum (in hundredths)	50.00%
Minimum years of service for employees to obtain maximum employer matching (in years)	P4Y
Employer matching rate for employee contributions maximum (in hundredths)	200.00%
Maximum years of service for employees to obtain minimum employer matching (in years)	15 years
Non-elective annual contribution for qualified employees	$ 750
Annual base employee compensation to qualify for non-elective employer contributions maximum	40,000
Employer discretionary contribution amount	662,000		738,000		802,000
Vesting period for employer contributions (in years)	5 years
Thrift Plan expenses	18,600,000		18,100,000		16,800,000
Incentive compensation plans [Abstract]
Expense related to cash-based incentive compensation plans	111,700,000		110,900,000		116,700,000
Pension Plan [Member]
Change in projected benefit obligation [Roll Forward]
Projected benefit obligation at beginning of year	44,765,000	[1],[2]	48,028,000
Interest cost	1,685,000		1,532,000
Actuarial loss (gain)	2,878,000		(1,543,000)
Benefits paid	(4,104,000)		(3,252,000)
Projected benefit obligation at end of year	45,224,000	[1],[2]	44,765,000	[1],[2]
Change in plan assets [Roll Forward]
Plan assets at fair value at beginning of year	48,812,000		45,920,000
Actual return on plan assets	4,735,000		6,144,000
Benefits paid	(4,104,000)		(3,252,000)
Plan assets at fair value at end of year	49,443,000		48,812,000
Defined Benefit Plan, Funded Status of Plan [Abstract]
Funded status of the plan	4,219,000		4,047,000
Components of net periodic benefit costs [Abstract]
Interest cost	1,685,000		1,532,000
Expected return on plan assets	(2,539,000)		(2,185,000)
Recognized prior service cost	1,175,000		1,175,000
Amortization of unrecognized net loss	2,584,000		3,830,000
Net periodic pension cost	555,000		2,002,000
Weighted-average assumptions [Abstract]
Discount rate (in hundredths)	3.42%		4.05%
Expected return on plan assets (in hundredths)	6.00%		6.00%
Estimated Future Benefit Payments [Abstract]
2015	3,535,000
2016	3,673,000
2017	3,270,000
2018	3,396,000
2019	3,628,000
Thereafter	44,057,000
Total estimated future benefit payments	61,559,000
Defined Benefit Plan, Information about Plan Assets [Abstract]
Inception to date return (in hundredths)	7.44%
Maximum allowable contributions to defined benefit plan	21,000,000
Net pension costs currently in accumulated other comprehensive income to be recognized as net periodic pension cost in next fiscal year	$ 267,000
Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Percentage of equities and bonds included in portfolio mix of fund used for defined benefit plan (in hundredths)	60.00%
Bonds [Member] | Pension Plan [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Percentage of equities and bonds included in portfolio mix of fund used for defined benefit plan (in hundredths)	40.00%

[1] Projected benefit obligation is based on January 1 measurement date.

[2] Projected benefit obligation equals accumulated benefit obligation.