Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 242,454	$ 218,512
Gross unrealized gains	8,388	4,327
Gross unrealized losses	(1,303)	(719)
Estimated fair value	249,539	222,120
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	55,576	80,732
Gross unrealized gains	1,289	1,358
Gross unrealized losses	(338)	(114)
Estimated fair value	56,527	81,976
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	71,689	70,426
Gross unrealized gains	4,558	2,009
Gross unrealized losses	(690)	(286)
Estimated fair value	75,557	72,149
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,692	1,675
Gross unrealized gains	21	0
Gross unrealized losses	0	(19)
Estimated fair value	1,713	1,656
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	113,497	65,679
Gross unrealized gains	2,520	960
Gross unrealized losses	(275)	(300)
Estimated fair value	$ 115,742	$ 66,339