Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative [Line Items]
Gain (Loss) on Discontinuation of Interest Rate Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net	$ 0	$ 0	$ 0	$ 0
Securities Purchased under Agreements to Resell	0		0		$ 0
Financial derivatives, at fair value	30,595		30,595		58,152
Derivative, Collateral [Abstract]
Collateral Pledged	25,079		25,079		25,643
Additional Collateral, Aggregate Fair Value	2,300		2,300		15,800
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,791,110		8,791,110		8,100,257
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	100		100		200
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0		0		0
Collateral Pledged	100		100		1,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	25,000		25,000		24,600
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	7,700,000		7,700,000		6,900,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,800,000		8,800,000		8,100,000
ERROR in label resolution. | Excluding netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	24,600		24,600		24,500
Financial derivatives, at fair value	61,100		61,100		65,700
ERROR in label resolution. | Netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	100		100		200
Financial derivatives, at fair value	$ 27,300		$ 27,300		$ 41,400