Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 item	Dec. 31, 2013	Dec. 31, 2012
Supplemental pension arrangements for key executives
Benefit Plans
Employee's eligibility age to receive a defined monthly benefit	65
Accrual for present value of future benefit payments	$ 5,009	$ 3,710
Expenses related to the plan	385	145	486
Deferred benefits for certain former key executives | Central
Benefit Plans
Accrual for present value of future benefit payments	4,225	3,586
Expenses related to the plan	1,060	565
Minimum period over which the annual payments will be made when the executives retire or upon death or disability	10 years
Face value of life insurance contracts	10,826	11,536
Cash surrender value of life insurance contracts	3,939	4,175
Deferred benefits for certain former key executives | Central | Minimum
Benefit Plans
Aggregate annual payments that will be made when executives retire or upon their death or disability	32
Deferred benefits for certain former key executives | Central | Maximum
Benefit Plans
Aggregate annual payments that will be made when executives retire or upon their death or disability	$ 144