Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 298,492	$ 325,421
Gross unrealized gains	602	1,521
Gross unrealized losses	(8,496)	(1,148)
Estimated fair value	290,598	325,794
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	36,282	31,882
Gross unrealized gains	74	157
Gross unrealized losses	(1,330)	(78)
Estimated fair value	35,026	31,961
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	68,720	71,174
Gross unrealized gains	313	614
Gross unrealized losses	(2,600)	(355)
Estimated fair value	66,433	71,433
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	1,721	1,706
Gross unrealized gains	0	16
Gross unrealized losses	(17)	0
Estimated fair value	1,704	1,722
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	191,769	220,659
Gross unrealized gains	215	734
Gross unrealized losses	(4,549)	(715)
Estimated fair value	$ 187,435	$ 220,678