Debt - Summary of Unsecured Note Balances, Interest Rate and Maturity Date Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Net Principal Balance	$ 5,038,812	$ 4,848,079
Fixed Rate Public Notes [Member]
Debt Instrument [Line Items]
Net Principal Balance	$ 4,591,373	$ 4,397,829
Weighted average interest rate	4.61%	4.90%
Debt instrument maturity year, start	2020	2017
Debt instrument maturity year, end	2047	2045
Fixed Rate Public Notes [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest Rate Ranges	2.85%	2.85%
Fixed Rate Public Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest Rate Ranges	7.57%	7.57%
Floating Rate Public Notes [Member]
Debt Instrument [Line Items]
Net Principal Balance	$ 447,439	$ 450,250
Weighted average interest rate	1.82%	1.28%
Debt instrument, maturity year	2019	2019