Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 383,215	$ 365,776
Gross unrealized gains	2,245	5,056
Gross unrealized losses	(7,330)	(2,999)
Estimated fair value	378,130	367,833
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	140,583	131,872
Gross unrealized gains	1,247	1,984
Gross unrealized losses	(2,162)	(1,090)
Estimated fair value	139,668	132,766
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	72,428	73,612
Gross unrealized gains	268	1,724
Gross unrealized losses	(1,618)	(227)
Estimated fair value	71,078	75,109
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,619	1,796
Gross unrealized gains	0	0
Gross unrealized losses	(70)	(40)
Estimated fair value	1,549	1,756
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	168,585	158,496
Gross unrealized gains	730	1,348
Gross unrealized losses	(3,480)	(1,642)
Estimated fair value	$ 165,835	$ 158,202