Summary of Significant Accounting Policies (Changes in AOCI Balances by Component) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Beginning balance	$ 1,440,142
Ending balance	1,553,856	$ 1,440,142
Accumulated Other Comprehensive Loss
Beginning balance	(42,100)	(45,449)	$ (42,718)
Other comprehensive loss before reclassifications	(6,124)	2,026	(4,254)
Amounts reclassified from AOCI	1,483	1,323	1,523
Ending balance	(46,741)	(42,100)	(45,449)
Defined Benefit Pension Plan
Beginning balance	(37,826)	(38,670)	(39,441)
Other comprehensive loss before reclassifications	(7,993)	(539)	(695)
Amounts reclassified from AOCI	1,732	1,383	1,466
Ending balance	(44,087)	(37,826)	(38,670)
Foreign Currency Translation
Beginning balance	(5,371)	(6,315)	(3,591)
Other comprehensive loss before reclassifications	49	944	(2,724)
Amounts reclassified from AOCI	0	0	0
Ending balance	(5,322)	(5,371)	(6,315)
Unrealized Gain (Loss) in Fair Value of Investments
Beginning balance	1,097	(464)	314
Other comprehensive loss before reclassifications	1,820	1,621	(835)
Amounts reclassified from AOCI	(249)	(60)	57
Ending balance	$ 2,668	$ 1,097	$ (464)