Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 372,979	$ 366,572
Gross unrealized gains	5,654	4,317
Gross unrealized losses	(2,333)	(4,286)
Estimated fair value	376,300	366,603
U.S. and foreign governments, agencies and municipalities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	134,134	136,316
Gross unrealized gains	2,024	1,571
Gross unrealized losses	(850)	(1,707)
Estimated fair value	135,308	136,180
Corporate notes and bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	76,105	69,376
Gross unrealized gains	1,888	1,180
Gross unrealized losses	(232)	(665)
Estimated fair value	77,761	69,891
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	1,792	1,568
Gross unrealized gains	0	0
Gross unrealized losses	(22)	(32)
Estimated fair value	1,770	1,536
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	160,948	159,312
Gross unrealized gains	1,742	1,566
Gross unrealized losses	(1,229)	(1,882)
Estimated fair value	$ 161,461	$ 158,996