Guarantees and Collateral (Collateral and Restricted Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial Instruments Owned and Pledged as Collateral [Line Items]
Restricted cash	$ 5,668	$ 5,635
Bonds Available-for-Sale	70,001	85,347
Investments in partnerships	8,728	8,779
Total Assets Pledged	84,397	99,761
Debt and derivatives | SAWHF
Financial Instruments Owned and Pledged as Collateral [Line Items]
Restricted cash	1,348	1,340
Investments in partnerships	8,728	8,779
Total Assets Pledged	$ 10,076	$ 10,119
Ownership interest (as a percent)	11.85%	11.85%
Debt and derivatives TRSs
Financial Instruments Owned and Pledged as Collateral [Line Items]
Restricted cash	$ 4,312	$ 4,287
Bonds Available-for-Sale	70,001	85,347
Total Assets Pledged	74,313	89,634
Other
Financial Instruments Owned and Pledged as Collateral [Line Items]
Restricted cash	8	8
Total Assets Pledged	$ 8	$ 8