Derivative Financial Instruments - Notional Values (Details) - Interest rate swaps - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Notional values	$ 5,076,972	$ 5,124,951
Trading
Derivatives, Fair Value [Line Items]
Notional values	1,008,917	973,539
Cash Flow Hedges | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Notional values	1,110,612	1,106,920
Fair Value Hedges | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Notional values	$ 2,957,443	$ 3,044,492