Employee Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplemental Executive Retirement Plan Disclosure
Rabbi Trust balance in cash and cash equivalents	$ 397,697,000	$ 284,258,000	$ 159,399,000	$ 168,045,000
Supplemental Executive Retirement Plan (SERP)
Supplemental Executive Retirement Plan Disclosure
Eligibility age	60 years
Number of months of salary taken as gross base for receipt of monthly payments	36 months
Vesting period of plan, when participants begin receiving payments	6 months
Rabbi Trust balance in cash and cash equivalents	5,100,000	5,100,000
Reconciliation of beginning and ending balances of the projected benefit obligation
Projected benefit obligation at the beginning of the year	51,034,000	47,206,000
Service cost	5,517,000	5,406,000	4,315,000
Interest cost	2,367,000	1,584,000	1,475,000
Plan amendments	3,862,000
Actuarial gain	(4,825,000)	(3,162,000)
Projected benefit obligation at the end of the year	57,955,000	51,034,000	47,206,000
Unfunded at end of the year	57,955,000	51,034,000
Accumulated benefit obligation	43,500,000	37,200,000
Current benefit obligation	20,900,000
Future estimated benefit payments
2015	20,875,000
2019	4,430,000
2020-2024	36,605,000
Total future estimated benefit payments	$ 61,910,000