Commitments and Contingencies - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Commitments And Contingencies Disclosure [Line Items]
Additional investment commitment - other investments	$ 107,300,000
Time period over which commitment exists - limited partnerships	7 years 8 months 12 days
Additional investment commitment - investments in affiliates	112,800,000
Time period over which commitment exists, investments in affiliates	6 years 10 months 24 days
Maximum percentage tax on branch profits, contingent	30.00%
Total rent expense under operating leases	36,600,000		38,700,000		35,600,000
Capital lease asset recognized	82,800,000		94,400,000
Deferred gain on sale/leaseback transaction	25,200,000		28,700,000
Future minimum lease payments	191,364,000
Shareholders' equity | Credit concentration risk
Commitments And Contingencies Disclosure [Line Items]
Maximum percentage of aggregate direct investment in any single corporate issuer	5.00%
Letter of credit facility
Commitments And Contingencies Disclosure [Line Items]
Letters of credit, amount outstanding	$ 1,790,561,000	[1],[2]	$ 1,895,425,000	[1],[2]
Percentage of letters of credit collateralized by investment portfolios	66.20%	[1]	67.60%	[1]

[1] At December 31, 2014, there were eight available letter of credit facilities; at December 31, 2013, there were seven available letter of credit facilities.

[2] “In Use” and “Outstanding” data represent December 31, 2014 and 2013 accreted values.