Commitments and Contingencies (Details) - Narrative (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010 contracts
Commitments And Contingencies Disclosure [Line Items]
Additional investment commitment - other investments		$ 92,400,000
Time period over which commitment exists - limited partnerships		9 years
Additional investment commitment - investments in affiliates		49,700,000
Time period over which commitment exists, investments in affiliates		5 years
Maximum percentage tax on branch profits, contingent		30.00%
Letters of credit outstanding at end of period		1,800,000,000	1,900,000,000
Percentage of letters of credit collateralized by investment portfolios		93.30%	93.80%
Number of financial guarantee contracts				37
Notional financial guarantee exposure eliminated by commutation				41,900,000
Payment made to eliminate financial guarantee through commutation	22,100,000			38,000,000
Principal outstanding				900,000,000
Payment received on settlement of financial guarantee				15,000,000
Capital lease asset recognized		98,500,000	100,700,000
Deferred gain on sale/leaseback transaction		29,900,000	30,600,000
Future minimum lease payments		222,415,000
Total rent expense under operating leases		35,600,000	32,900,000	31,800,000
Commuted
Commitments And Contingencies Disclosure [Line Items]
Number of financial guarantee contracts				32
Terminated
Commitments And Contingencies Disclosure [Line Items]
Number of financial guarantee contracts			1
Payment made to eliminate financial guarantee through commutation				$ 500,000
Non-performing
Commitments And Contingencies Disclosure [Line Items]
Number of financial guarantee contracts			3
Shareholders' equity | Credit concentration risk
Commitments And Contingencies Disclosure [Line Items]
Maximum percentage of aggregate direct investment in any single corporate issuer		5.00%