Business Segments (Principal Geographical Areas) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Principal Geographical Areas Information
Revenue	$ 1,349,516	$ 1,442,091	$ 1,276,427	$ 1,250,729	$ 1,177,725	$ 1,189,345	$ 1,125,275	$ 958,487	$ 5,318,763	$ 4,450,832	$ 4,454,662
Assets	5,045,617				4,485,777				5,045,617	4,485,777
United States
Principal Geographical Areas Information
Revenue									4,953,045	4,073,691	4,180,206
Assets	4,836,735				4,271,722				4,836,735	4,271,722
Foreign and U.S. Territories
Principal Geographical Areas Information
Revenue									365,718	377,141	$ 274,456
Assets	$ 208,882				$ 214,055				$ 208,882	$ 214,055