Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Derivative [Line Items]
Financial derivatives, at fair value	$ 69,373		$ 69,373		$ 84,844
Derivative, Collateral [Abstract]
Collateral Pledged	35,335		35,335		46,627
Additional Collateral, Aggregate Fair Value	38,700		38,700		46,800
Notional Disclosures [Abstract]
Derivative, Notional Amount	7,542,090		7,542,090		6,837,571
Excluding netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	10,800		10,800		6,100
Financial derivatives, at fair value	83,200		83,200		99,400
Netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	700		700		400
Financial derivatives, at fair value	74,000		74,000		93,400
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	700		700		400
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0		0		0
Collateral Pledged	35,300		35,300		46,600
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	0		0		0
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	4,400,000		4,400,000		4,000,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	7,500,000		7,500,000		$ 6,800,000
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	$ (200)	$ 100	$ 100	$ (80)