Derivative Instruments (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative, Notional Amount	$ 3,254,921	$ 1,963,178
Interest Rate Floor [Member]
Derivative [Line Items]
Derivative, Notional Amount	1,000,000	0
Premium paid for interest rate floor cash flow hedge	20,700
Net unrealized gain (loss) on interest rate floors	9,100
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	(2,800)
Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative, Notional Amount	2,006,280	1,741,412
Variation Margin Impact to Positive Fair Values of Cleared Swaps	(8,100)	(4,500)
Variation Margin Impact to Negative Fair Values of Cleared Swaps	$ (6,500)	$ (4,300)