Income Taxes (Summary of Deferred Income Tax Asset Valuation Allowance) (Details) - Deferred income tax asset valuation allowance - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 109,643	$ 95,398	$ 80,796
Additions (reductions) charged to expense	13,470	14,245	14,602
Other	0	0	0
Balance at end of year	$ 123,113	$ 109,643	$ 95,398