Summary of Significant Accounting Policies Recent accounting change (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Policy Acquisition Costs	$ 404,047								$ 364,937								$ 404,047	$ 364,937
Deferred tax liabilities	507,296								435,897								507,296	435,897
Stockholders' Equity Attributable to Parent	4,306,217								3,953,356								4,306,217	3,953,356
Other Cost and Expense, Operating																	1,799,623	1,626,526	1,500,757
Income before income taxes																	701,928	513,086	598,910
Income Tax Expense (Benefit)																	(191,285)	(121,945)	(152,226)
Net income to common stockholders	165,489		100,947		108,838		135,318		117,027		76,410		82,184		115,590		510,592	391,211	446,405
Earnings Per Share, Basic	$ 1.22	[1]	$ 0.74	[1]	$ 0.79	[1]	$ 0.98	[1]	$ 0.85	[1]	$ 0.55	[1]	$ 0.58	[1]	$ 0.82	[1]	$ 3.72	$ 2.80	$ 3.00
Earnings Per Share, Diluted	$ 1.17	[1]	$ 0.71	[1]	$ 0.76	[1]	$ 0.94	[1]	$ 0.82	[1]	$ 0.53	[1]	$ 0.56	[1]	$ 0.78	[1]	$ 3.56	$ 2.69	$ 2.88
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Policy Acquisition Costs									448,795									448,795
Deferred tax liabilities									31,623									31,623
Stockholders' Equity Attributable to Parent									4,008,426									4,008,426
Other Cost and Expense, Operating																		1,621,329	1,496,362
Income before income taxes																		518,283	603,305
Income Tax Expense (Benefit)																		123,550	153,739
Net income to common stockholders																		394,803	449,287
Earnings Per Share, Basic																		$ 2.83	$ 3.02
Earnings Per Share, Diluted																		$ 2.71	$ 2.90
Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Policy Acquisition Costs									364,937									364,937
Deferred tax liabilities									2,835									2,835
Stockholders' Equity Attributable to Parent									3,953,356									3,953,356
Other Cost and Expense, Operating																		1,626,526	1,500,757
Income before income taxes																		513,086	598,910
Income Tax Expense (Benefit)																		121,945	152,226
Net income to common stockholders																		$ 391,211	$ 446,405
Earnings Per Share, Basic																		$ 2.80	$ 3.00
Earnings Per Share, Diluted																		$ 2.69	$ 2.88

[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.