Summary of Significant Accounting Policies (Recent accounting change) (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011 Accounting Standards Update 2010-26 [Member]	Dec. 31, 2011 Accounting Standards Update 2010-26 [Member] Previously Reported	Dec. 31, 2009 Accounting Standards Update 2010-26 [Member] Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Period Increase (Decrease)																						$ 49,000,000
Other operating costs and expenses																	2,000,684,000	1,799,623,000	1,626,526,000	1,626,526,000	1,621,329,000
Income before income taxes																	698,888,000	701,928,000	513,086,000	513,086,000	518,283,000
Federal and foreign income taxes																	(193,587,000)	(191,285,000)	(121,945,000)	(121,945,000)	(123,550,000)
Net income	$ 130,379,000		$ 136,974,000		$ 115,957,000		$ 116,615,000		$ 165,489,000		$ 100,947,000		$ 108,838,000		$ 135,318,000		$ 499,925,000	$ 510,592,000	$ 391,211,000	$ 391,211,000	$ 394,803,000
Basic net income per share (dollars per share)	$ 0.97	[1]	$ 1.01	[1]	$ 0.85	[1]	$ 0.86	[1]	$ 1.22	[1]	$ 0.74	[1]	$ 0.79	[1]	$ 0.98	[1]	$ 3.69	$ 3.72	$ 2.80	$ 2.80	$ 2.83
Diluted net income per share (dollars per share)	$ 0.93	[1]	$ 0.97	[1]	$ 0.82	[1]	$ 0.83	[1]	$ 1.17	[1]	$ 0.71	[1]	$ 0.76	[1]	$ 0.94	[1]	$ 3.55	$ 3.56	$ 2.69	$ 2.69	$ 2.71

[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.