Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 235,256	$ 270,313
Gross unrealized gains	5,122	218
Gross unrealized losses	(724)	(5,202)
Estimated fair value	239,654	265,329
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	88,050	109,776
Gross unrealized gains	1,946	47
Gross unrealized losses	(47)	(1,336)
Estimated fair value	89,949	108,487
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	71,763	58,714
Gross unrealized gains	1,907	4
Gross unrealized losses	(338)	(1,780)
Estimated fair value	73,332	56,938
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,666	1,637
Gross unrealized gains	0	0
Gross unrealized losses	(11)	(67)
Estimated fair value	1,655	1,570
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	73,777	100,186
Gross unrealized gains	1,269	167
Gross unrealized losses	(328)	(2,019)
Estimated fair value	$ 74,718	$ 98,334