Summary of Significant Accounting Policies (Recent accounting change) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						6 Months Ended			9 Months Ended			12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015		Mar. 31, 2015		Jun. 30, 2016	Jun. 30, 2015		Sep. 30, 2016	Sep. 30, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other operating costs and expenses													$ 2,395,619	$ 2,289,750	$ 2,157,456
Income before income taxes													896,438	732,030	952,196
Federal and foreign income taxes	$ 302,593												(292,953)	(227,923)	(302,593)
Net income	$ 152,790	$ 119,511	$ 109,745		$ 118,307		$ 108,967	$ 123,035		$ 220,650	$ 152,607		$ 601,916	$ 503,694	$ 648,884
Basic net income per share (dollars per share)	$ 1.26	$ 0.97	$ 0.89	[1]	$ 0.94	[1]	$ 0.89	$ 0.99	[1]	$ 1.80	$ 1.24	[1]	$ 4.91	$ 4.06	$ 5.07
Diluted net income per share (dollars per share)	$ 1.20	$ 0.93	$ 0.85	[1]	$ 0.89	[1]	$ 0.85	$ 0.95	[1]	$ 1.72	$ 1.18	[1]	$ 4.68	$ 3.87	$ 4.86

[1] Net income per share (“EPS”) in each quarter is computed using the weighted-average number of shares outstanding during that quarter, while EPS for the full year is computed using the weighted-average number of shares outstanding during the year. Thus, the sum of the four quarters EPS does not necessarily equal the full-year EPS.