Long-Term Incentive and Supplemental Executive Retirement Plans (Details 2) - Supplemental Executive Retirement plan - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Employee benefit plan
Value of assets held in trust	$ 14,400,000		$ 14,400,000		$ 15,700,000
Costs recognized and benefits paid
Interest costs	142,000	$ 173,000	328,000	$ 294,000
Amortization of unrecognized actuarial loss	7,000	7,000	117,000	15,000
Total costs	149,000	180,000	445,000	309,000
Benefits paid	0	0	1,477,000	90,000
Investment gain (loss)	(73,000)	$ 418,000	119,000	$ 533,000
Projected benefit obligations	$ 14,900,000		$ 14,900,000		$ 16,100,000
Weighted-average discount rate (as a percent)	1.00%	2.00%	1.00%	2.00%
Expected future payouts
2016	$ 5,400,000		$ 5,400,000
2017	1,600,000		1,600,000
2018	2,900,000		2,900,000
2019	1,500,000		1,500,000
2020	90,000		90,000
2021 through 2025	$ 4,600,000		$ 4,600,000