Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 221,513	$ 270,313
Gross unrealized gains	3,288	218
Gross unrealized losses	(672)	(5,202)
Estimated fair value	224,129	265,329
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	85,292	109,776
Gross unrealized gains	1,063	47
Gross unrealized losses	(47)	(1,336)
Estimated fair value	86,308	108,487
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	49,507	58,714
Gross unrealized gains	1,320	4
Gross unrealized losses	(53)	(1,780)
Estimated fair value	50,774	56,938
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,656	1,637
Gross unrealized gains	0	0
Gross unrealized losses	(24)	(67)
Estimated fair value	1,632	1,570
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	85,058	100,186
Gross unrealized gains	905	167
Gross unrealized losses	(548)	(2,019)
Estimated fair value	$ 85,415	$ 98,334