Fair Value Measurements and Derivative Instruments (Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 365,776	$ 366,572
Gross unrealized gains	5,056	4,317
Gross unrealized losses	(2,999)	(4,286)
Estimated fair value	367,833	366,603
Investment securities at a loss position for more than 12 months, aggregate unrealized holding losses, less than $1million for 2016	2,000	1,000
Investment securities at a loss position for more than 12 months, estimated fair value	116,000	12,000
Investment securities at a loss position for less than 12 months, unrealized holding losses	1,000	4,000
Investment securities at a loss position for less than 12 months, estimated fair value	91,000	171,000
Government and related securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	131,872	136,316
Gross unrealized gains	1,984	1,571
Gross unrealized losses	(1,090)	(1,707)
Estimated fair value	132,766	136,180
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	73,612	69,376
Gross unrealized gains	1,724	1,180
Gross unrealized losses	(227)	(665)
Estimated fair value	75,109	69,891
Commingled fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,796	1,568
Gross unrealized gains	0	0
Gross unrealized losses	(40)	(32)
Estimated fair value	1,756	1,536
Mortgage-backed / asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	158,496	159,312
Gross unrealized gains	1,348	1,566
Gross unrealized losses	(1,642)	(1,882)
Estimated fair value	$ 158,202	$ 158,996