Fair Value Measurements and Derivative Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 261,688	$ 218,512
Gross unrealized gains	8,561	4,327
Gross unrealized losses	(3,289)	(719)
Estimated fair value	266,960	222,120
Government and related securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	81,698	80,732
Gross unrealized gains	4,462	1,358
Gross unrealized losses	(360)	(114)
Estimated fair value	85,800	81,976
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	73,338	70,426
Gross unrealized gains	1,270	2,009
Gross unrealized losses	(2,596)	(286)
Estimated fair value	72,012	72,149
Commingled fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,684	1,675
Gross unrealized gains	7	0
Gross unrealized losses	0	(19)
Estimated fair value	1,691	1,656
Mortgage-backed / asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	104,968	65,679
Gross unrealized gains	2,822	960
Gross unrealized losses	(333)	(300)
Estimated fair value	$ 107,457	$ 66,339