Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 371,828	$ 365,776
Gross unrealized gains	1,767	5,056
Gross unrealized losses	(8,517)	(2,999)
Estimated fair value	365,078	367,833
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	136,967	131,872
Gross unrealized gains	980	1,984
Gross unrealized losses	(2,879)	(1,090)
Estimated fair value	135,068	132,766
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	71,261	73,612
Gross unrealized gains	218	1,724
Gross unrealized losses	(1,555)	(227)
Estimated fair value	69,924	75,109
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,628	1,796
Gross unrealized gains	0	0
Gross unrealized losses	(82)	(40)
Estimated fair value	1,546	1,756
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	161,972	158,496
Gross unrealized gains	569	1,348
Gross unrealized losses	(4,001)	(1,642)
Estimated fair value	$ 158,540	$ 158,202