Summary of Significant Accounting Policies (Recent accounting change) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other operating costs and expenses													$ 2,289,750	$ 2,157,456	$ 2,000,684
Income before income taxes													732,030	952,196	698,888
Federal and foreign income taxes													(227,923)	(302,593)	(193,587)
Net income	$ 109,745	$ 152,607	$ 123,035	$ 118,307	$ 110,711		$ 188,539		$ 179,961		$ 169,673		$ 503,694	$ 648,884	$ 499,925
Basic net income per share (dollars per share)	$ 0.89	$ 1.24	$ 0.99	$ 0.94	$ 0.87	[1]	$ 1.48	[1]	$ 1.41	[1]	$ 1.31	[1]	$ 4.06	$ 5.07	$ 3.69
Diluted net income per share (dollars per share)	$ 0.85	$ 1.18	$ 0.95	$ 0.89	$ 0.83	[1]	$ 1.42	[1]	$ 1.35	[1]	$ 1.25	[1]	$ 3.87	$ 4.86	$ 3.55

[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.