Derivative Financial Instruments - Notional Values (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	$ 6,000,000
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	5,996,310	$ 5,459,075
Interest Rate Swaps | Trading
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	1,577,978	987,577
Interest Rate Swaps | Cash Flow | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	1,280,367	1,408,649
Interest Rate Swaps | Fair Value | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Notional Values	$ 3,137,965	$ 3,062,849