Commitments, Contingencies and Guarantees (Guarantees Issued) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Insurance Treaty Guarantee [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 1,500,220	$ 1,392,352
Insurance Treaty Guarantee Net Of Assets Held In Trust [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	1,402,737	1,291,445
Guarantee Of Borrowed Securities [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	274,555	269,980
Product Financing Arrangement [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	50,666	61,273
Guarantee Of Lease Obligations [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 156	$ 392