Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 352,198	$ 357,339
Gross unrealized gains	6,709	6,836
Gross unrealized losses	(2,419)	(2,133)
Estimated fair value	356,488	362,042
U.S. Government, federal agencies and municipalities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	118,152	135,839
Gross unrealized gains	2,558	2,905
Gross unrealized losses	(951)	(764)
Estimated fair value	119,759	137,980
Corporate notes and bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	66,537	66,170
Gross unrealized gains	1,235	1,569
Gross unrealized losses	(644)	(291)
Estimated fair value	67,128	67,448
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	167,509	155,330
Gross unrealized gains	2,916	2,362
Gross unrealized losses	(824)	(1,078)
Estimated fair value	$ 169,601	$ 156,614