Investments Equity Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Other investments	$ 469,438	$ 391,217
Insurance Linked Redemption Cash Received [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	24,804
Real Estate [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	15,831	16,123
Interest Rate Contract [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	233,904	177,357
Illiquid Investments [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	38,385	84,297
Insurance Linked [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	27,752	29,318
Insurance Linked [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	24,698
Equity [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	50,691	43,954
Credit Index Product [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	60,000
Equity investments in pooled funds [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	426,563	351,049
Readily determinable fair value [Member] | Interest Rate Contract [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	78,998	$ 27,154
Commitments [Member] | Partnership Interest [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	$ 25,000