Derivative Financial Instruments - Notional Values (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Notional values	$ 6,500,000
Interest rate swaps
Derivatives, Fair Value [Line Items]
Notional values	6,529,489	$ 5,459,075
Interest rate swaps | Trading
Derivatives, Fair Value [Line Items]
Notional values	1,317,494	987,577
Interest rate swaps | Cash Flow Hedges | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Notional values	1,344,791	1,408,649
Interest rate swaps | Fair Value Hedges | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Notional values	$ 3,867,204	$ 3,062,849