Supplemental Executive Retirement Plan (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Supplemental Executive Retirement Plan Disclosure
Cash and cash equivalents	$ 254,359	$ 397,697	$ 259,046	$ 278,889
Supplemental Executive Retirement Plan (SERP)
Supplemental Executive Retirement Plan Disclosure
Cash and cash equivalents	$ 5,100	$ 5,100