Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 398,054	$ 218,512
Gross unrealized gains	1,063	4,327
Gross unrealized losses	(4,643)	(719)
Estimated fair value	394,474	222,120
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	35,018	80,732
Gross unrealized gains	144	1,358
Gross unrealized losses	(719)	(114)
Estimated fair value	34,443	81,976
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	84,457	70,426
Gross unrealized gains	422	2,009
Gross unrealized losses	(1,920)	(286)
Estimated fair value	82,959	72,149
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,699	1,675
Gross unrealized gains	21	0
Gross unrealized losses	0	(19)
Estimated fair value	1,720	1,656
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	276,880	65,679
Gross unrealized gains	476	960
Gross unrealized losses	(2,004)	(300)
Estimated fair value	$ 275,352	$ 66,339