Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 363,187	$ 366,572
Gross unrealized gains	5,697	4,317
Gross unrealized losses	(2,790)	(4,286)
Estimated fair value	366,094	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	124,051	136,316
Gross unrealized gains	2,142	1,571
Gross unrealized losses	(1,092)	(1,707)
Estimated fair value	125,101	136,180
Corporate notes and bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	75,876	69,376
Gross unrealized gains	1,779	1,180
Gross unrealized losses	(303)	(665)
Estimated fair value	77,352	69,891
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	1,584	1,568
Gross unrealized gains	0	0
Gross unrealized losses	(24)	(32)
Estimated fair value	1,560	1,536
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	161,676	159,312
Gross unrealized gains	1,776	1,566
Gross unrealized losses	(1,371)	(1,882)
Estimated fair value	$ 162,081	$ 158,996