Derivative Instruments Derivative Instruments (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount of Derivatives	$ 3,956,374	$ 3,254,921
Interest Rate Floor [Member]
Derivative [Line Items]
Notional Amount of Derivatives	1,500,000	1,000,000
Premium paid for interest rate floor cash flow hedge	$ 31,300
Maximum Length of Time Hedged in Interest Rate Cash Flow Hedge	7 years
Unrealized gain (loss) on interest rate cash flow hedges, net of tax, amount recorded in accumulated other comprehensive income	$ 39,300
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ (4,100)
Interest Rate Floor [Member] | Interest Rate Floor 1 [Member]
Derivative [Line Items]
Derivative, Floor Interest Rate	2.25%
Derivative, Effective Date	Jan. 01, 2020
Derivative, Maturity Date	Jan. 01, 2026
Interest Rate Floor [Member] | Interest Rate Floor 2 [Member]
Derivative [Line Items]
Derivative, Floor Interest Rate	2.50%
Derivative, Effective Date	Jun. 01, 2020
Derivative, Maturity Date	Jun. 01, 2026
Interest Rate Floor [Member] | Interest Rate Floor 3 [Member]
Derivative [Line Items]
Derivative, Floor Interest Rate	2.00%
Derivative, Effective Date	Dec. 15, 2020
Derivative, Maturity Date	Dec. 15, 2026
Interest Rate Swaps [Member]
Derivative [Line Items]
Notional Amount of Derivatives	$ 2,182,949	2,006,280
Variation Margin Impact to Positive Fair Values of Cleared Swaps	(372)	(8,100)
Variation Margin Impact to Negative Fair Values of Cleared Swaps	$ (27,900)	$ (6,500)