Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 238,733	$ 270,313
Gross unrealized gains	885	218
Gross unrealized losses	(2,246)	(5,202)
Estimated fair value	237,372	265,329
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	94,831	109,776
Gross unrealized gains	221	47
Gross unrealized losses	(575)	(1,336)
Estimated fair value	94,477	108,487
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	51,769	58,714
Gross unrealized gains	329	4
Gross unrealized losses	(421)	(1,780)
Estimated fair value	51,677	56,938
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,647	1,637
Gross unrealized gains	0	0
Gross unrealized losses	(47)	(67)
Estimated fair value	1,600	1,570
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	90,486	100,186
Gross unrealized gains	335	167
Gross unrealized losses	(1,203)	(2,019)
Estimated fair value	$ 89,618	$ 98,334