Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 380,474	$ 365,776
Gross unrealized gains	2,827	5,056
Gross unrealized losses	(6,103)	(2,999)
Estimated fair value	377,198	367,833
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	142,719	131,872
Gross unrealized gains	1,393	1,984
Gross unrealized losses	(1,996)	(1,090)
Estimated fair value	142,116	132,766
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	72,790	73,612
Gross unrealized gains	555	1,724
Gross unrealized losses	(1,056)	(227)
Estimated fair value	72,289	75,109
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,791	1,796
Gross unrealized gains	0	0
Gross unrealized losses	(67)	(40)
Estimated fair value	1,724	1,756
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	163,174	158,496
Gross unrealized gains	879	1,348
Gross unrealized losses	(2,984)	(1,642)
Estimated fair value	$ 161,069	$ 158,202