Fair Value Measurements and Derivative Instruments (Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 366,572	$ 355,226
Gross unrealized gains	4,317	4,528
Gross unrealized losses	(4,286)	(3,835)
Estimated fair value	366,603	355,919
Investment securities at a loss position for more than 12 months, aggregate unrealized holding losses, less than $1million for 2016	1,000	2,000
Investment securities at a loss position for more than 12 months, estimated fair value	12,000	36,000
Investment securities at a loss position for less than 12 months, unrealized holding losses	4,000	2,000
Investment securities at a loss position for less than 12 months, estimated fair value	171,000	146,000
U.S. and foreign governments, agencies and municipalities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	136,316	114,265
Gross unrealized gains	1,571	1,804
Gross unrealized losses	(1,707)	(1,268)
Estimated fair value	136,180	114,801
Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	69,376	63,140
Gross unrealized gains	1,180	823
Gross unrealized losses	(665)	(1,079)
Estimated fair value	69,891	62,884
Commingled fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,568
Gross unrealized gains	0
Gross unrealized losses	(32)
Estimated fair value	1,536
Mortgage-backed / asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	159,312	177,821
Gross unrealized gains	1,566	1,901
Gross unrealized losses	(1,882)	(1,488)
Estimated fair value	$ 158,996	$ 178,234