Derivative Financial Instruments - Notional Values (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	$ 5,500,000
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	5,459,075	$ 5,177,388
Interest Rate Swaps | Trading
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	987,577	494,638
Interest Rate Swaps | Cash Flow | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	1,408,649	1,054,688
Interest Rate Swaps | Fair Value | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	$ 3,062,849	$ 3,628,062