Business Segments (Principal Geographical Areas) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Principal Geographical Areas Information
Revenues	$ 1,193,068	$ 1,199,505	$ 1,247,274	$ 1,117,361	$ 1,246,599	$ 1,332,978	$ 1,308,130	$ 1,085,369	$ 4,757,208	$ 4,973,076	$ 4,920,472
Assets	4,264,123				4,038,620				4,264,123	4,038,620
United States
Principal Geographical Areas Information
Revenues									4,613,644	4,802,393	4,694,165
Assets	4,093,673				3,911,865				4,093,673	3,911,865
Foreign and U.S. Territories [Member]
Principal Geographical Areas Information
Revenues									143,564	170,683	$ 226,307
Assets	$ 170,450				$ 126,755				$ 170,450	$ 126,755