Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 356,132	$ 355,226
Gross unrealized gains	8,235	4,528
Gross unrealized losses	(1,861)	(3,835)
Estimated fair value	362,506	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	129,918	114,265
Gross unrealized gains	3,328	1,804
Gross unrealized losses	(321)	(1,268)
Estimated fair value	132,925	114,801
Corporate notes and bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	61,181	63,140
Gross unrealized gains	1,869	823
Gross unrealized losses	(379)	(1,079)
Estimated fair value	62,671	62,884
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	1,542
Gross unrealized gains	14
Gross unrealized losses	0
Estimated fair value	1,556
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	163,491	177,821
Gross unrealized gains	3,024	1,901
Gross unrealized losses	(1,161)	(1,488)
Estimated fair value	$ 165,354	$ 178,234