Commitments and Contingencies - Narrative (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2016 USD ($) Guarantee_contracts	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Additional investment commitment - other investments		$ 59,300
Time period over which commitment exists - limited partnerships		4 years 9 months 18 days
Additional investment commitment - investments in affiliates		$ 237,100
Time period over which commitment exists, investments in affiliates		4 years 10 months 24 days
Maximum percentage tax on branch profits, contingent		30.00%
Total rent expense under operating leases		$ 68,000	$ 71,100	$ 36,600
Capital lease asset recognized		55,500	73,400
Capital lease liability		89,900	113,800
Deferred gain on sale/leaseback transaction		16,900	$ 22,300
Future minimum lease payments		131,289
Total gross claim liability recorded		$ 0
Weighted average contractual term to maturity in years		23 years 2 months 12 days	24 years 2 months 12 days
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]	$ 2,345,293	$ 2,515,653
Percentage of letters of credit collateralized by investment portfolios	[1]	48.60%	50.90%
Financial Guarantee Related to Securities Including Some Issued By European Financials [Member]
Commitments And Contingencies Disclosure [Line Items]
Number Of Financial Guarantee Contracts | Guarantee_contracts		2

[1] As of December 31, 2016 and December 31, 2015, there were fifteen available credit facilities.