Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 360,863	$ 325,421
Gross unrealized gains	645	1,521
Gross unrealized losses	(11,921)	(1,148)
Estimated fair value	349,587	325,794
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	51,411	31,882
Gross unrealized gains	42	157
Gross unrealized losses	(1,377)	(78)
Estimated fair value	50,076	31,961
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	69,913	71,174
Gross unrealized gains	300	614
Gross unrealized losses	(4,263)	(355)
Estimated fair value	65,950	71,433
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	1,712	1,706
Gross unrealized gains	0	16
Gross unrealized losses	(14)	0
Estimated fair value	1,698	1,722
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	237,827	220,659
Gross unrealized gains	303	734
Gross unrealized losses	(6,267)	(715)
Estimated fair value	$ 231,863	$ 220,678