Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 361,522	$ 357,339
Gross unrealized gains	5,399	6,836
Gross unrealized losses	(3,041)	(2,133)
Estimated fair value	363,880	362,042
U.S. Government, federal agencies and municipalities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	131,031	135,839
Gross unrealized gains	1,927	2,905
Gross unrealized losses	(1,244)	(764)
Estimated fair value	131,714	137,980
Corporate notes and bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	65,396	66,170
Gross unrealized gains	1,112	1,569
Gross unrealized losses	(668)	(291)
Estimated fair value	65,840	67,448
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	165,095	155,330
Gross unrealized gains	2,360	2,362
Gross unrealized losses	(1,129)	(1,078)
Estimated fair value	$ 166,326	$ 156,614