Derivative Instruments Derivative Instruments (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount of Derivatives	$ 2,672,156	$ 1,963,178
Interest Rate Floor [Member]
Derivative [Line Items]
Notional Amount of Derivatives	500,000	0
Premium paid for interest rate floor cash flow hedge	$ 9,500
Derivative, Maturity Date	Jan. 01, 2026
Unrealized gain (loss) on interest rate cash flow hedges, net of tax, amount recorded in accumulated other comprehensive income	$ (1,400)
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	(1,300)
Interest Rate Swaps [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,922,172	1,741,412
Variation Margin Impact to Positive Fair Values of Cleared Swaps	(19,100)	(4,500)
Variation Margin Impact to Negative Fair Values of Cleared Swaps	$ (687)	$ (4,300)