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, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Net income (loss)	$ 1,654,341	$ 315,278	$ 621,861
Book/tax differences	(1,076,060)	(61,133)	(316,097)
Taxable income before adjustments	578,281	254,145	305,764
Less: capital gains	(441,836)	(63,550)	(294,901)
Adjusted taxable income subject to 90% distribution requirement	$ 136,445	$ 190,595	$ 10,863