Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	$ 332,258	$ 325,421
Gross unrealized gains	711	1,521
Gross unrealized losses	(6,543)	(1,148)
Estimated fair value	326,426	325,794
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	51,055	31,882
Gross unrealized gains	107	157
Gross unrealized losses	(636)	(78)
Estimated fair value	50,526	31,961
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	69,475	71,174
Gross unrealized gains	357	614
Gross unrealized losses	(2,158)	(355)
Estimated fair value	67,674	71,433
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	1,716	1,706
Gross unrealized gains	0	16
Gross unrealized losses	(10)	0
Estimated fair value	1,706	1,722
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized cost	210,012	220,659
Gross unrealized gains	247	734
Gross unrealized losses	(3,739)	(715)
Estimated fair value	$ 206,520	$ 220,678