Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
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	123,796		123,796		77,199
Derivative, Collateral [Abstract]
Collateral Pledged	89,732		89,732		37,986
Additional Collateral, Aggregate Fair Value	27,900		27,900		45,200
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,261,539		8,261,539		8,429,964
Excluding netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	12,100		12,100		6,400
Financial derivatives, at fair value	132,300		132,300		90,100
Netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	0		0		0
Financial derivatives, at fair value	117,600		117,600		83,200
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	0		0		0
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0		0		0
Collateral Pledged	89,700		89,700		38,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	0		0		0
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	7,300,000		7,300,000		6,200,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,200,000		$ 8,200,000		$ 8,400,000