Investments Investments - Equity Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Other investments	$ 462,427	$ 391,217
Real Estate [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	15,321	16,123
Interest Rate Contract [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	234,758	177,357
Illiquid Investments [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	53,445	84,297
Insurance Linked [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	3,179	29,318
Equity [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	50,342	43,954
Credit Index Product [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	61,095
Equity investments in pooled funds [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	418,140	351,049
Readily determinable fair value [Member] | Interest Rate Contract [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	50,342	27,154
Priced Through Pricing Vendors [Member] | Equity [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	80,050	$ 43,954
Commitments [Member] | Partnership Interest [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	9,827
Commitments [Member] | Capital of Dislocations [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	$ 50,000