Summary of Significant Accounting Policies (Reconciliation of Net Income to Taxable Income Before Dividends Paid Deduction) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
REIT dividend distribution requirement percentage	90.00%
Net income (loss)	$ 302,760	$ 432,644	$ 387,268
Book/tax differences	39,566	(120,421)	(97,079)
Taxable income before adjustments	342,326	312,223	290,189
Less: capital gains	(90,242)	(62,513)	(63,151)
Adjusted taxable income subject to 90% distribution requirement	$ 252,084	$ 249,710	$ 227,038