Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 350,981	$ 355,226
Gross unrealized gains	10,521	4,528
Gross unrealized losses	(1,061)	(3,835)
Estimated fair value	360,441	355,919
U.S. and foreign governments, agencies and municipalities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	109,027	114,265
Gross unrealized gains	4,145	1,804
Gross unrealized losses	(261)	(1,268)
Estimated fair value	112,911	114,801
Corporate notes and bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	79,264	63,140
Gross unrealized gains	2,982	823
Gross unrealized losses	(97)	(1,079)
Estimated fair value	82,149	62,884
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	1,559
Gross unrealized gains	16
Gross unrealized losses	0
Estimated fair value	1,575
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	161,131	177,821
Gross unrealized gains	3,378	1,901
Gross unrealized losses	(703)	(1,488)
Estimated fair value	$ 163,806	$ 178,234