Commitments, Contingencies and Guarantees (Guarantees Issued) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Insurance Treaty Guarantee [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 1,764,146	$ 1,392,352
Insurance Treaty Guarantee Net Of Assets Held In Trust [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	902,008	1,291,445
Guarantee Of Borrowed Securities [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	273,295	269,980
Product Financing Arrangement [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	41,246	61,273
Guarantee Of Lease Obligations [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 0	$ 392