Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments Net Asset Value		$ 322,900	$ 518,100
Spread on US Treasury rate for determining discount rate for fair value of deposit liabilities		0.50%	0.265%
Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments - structured transactions		$ 142,271	$ 141,329
Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	[1]	(77,700)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	[1]	15,500
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	[1]		(2,900)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	[1]		3,100
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	[1]	(5,000)	(34,100)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	[1]	3,000	$ 13,300
Non-Financial Services Sector | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		(1,900)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		$ 9,700

[1] Net transfers include both movement in and movement out of Level 3. Corporate - Non-Financials securities for 2016 includes $9.7 million of incoming transfers partially offset by $1.9 million outgoing transfers. Other asset-backed securities for 2016 includes $5.0 million of outgoing transfers partially offset by $3.0 million of incoming transfers, and 2015 includes $34.1 million of outgoing transfers partially offset by $13.3 million of incoming transfers. Other investments for 2016 includes $77.7 million of outgoing transfers partially offset by $15.5 million of incoming transfers. RMBS-Agency for 2015 includes $3.1 million of incoming transfers partially offset by $2.9 million of outgoing transfers.