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, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Net income (loss)	$ 387,268	$ 1,654,341	$ 315,278
Book/tax differences	(97,218)	(1,073,552)	(61,133)
Taxable income before adjustments	290,050	580,789	254,145
Less: capital gains	(62,858)	(441,577)	(63,550)
Adjusted taxable income subject to 90% distribution requirement	$ 227,192	$ 139,212	$ 190,595