Income Taxes - Reconciliation of Statutory Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected tax at statutory rate	$ 186,274	$ 144,708	$ 77,151
Non-taxable REIT income	(170,811)	(131,112)	(67,084)
State and local tax expense - net of federal benefit	2,306	2,399	1,249
Change in valuation allowance	159	(845)	(624)
Tax credits/true-up	(5,956)	(312)	274
Remeasurement of deferred balances	(8,460)	0	0
Miscellaneous	113	1,009	182
Total provision	$ 3,625	$ 15,847	$ 11,148
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected tax at statutory rate	35.00%	35.00%	35.00%
Non-taxable REIT income	(32.10%)	(31.70%)	(30.40%)
State and local tax expense - net of federal benefit	0.40%	0.60%	0.60%
Change in valuation allowance	0.00%	(0.20%)	(0.30%)
Tax credits/true-up	(1.10%)	(0.10%)	0.10%
Remeasurement of deferred balances	(1.60%)	0.00%	0.00%
Miscellaneous	0.00%	0.20%	0.10%
Total provision	0.60%	3.80%	5.10%