Corporate Indebtedness (Details) - USD ($)	1 Months Ended				12 Months Ended
	Jul. 31, 2015	Jan. 31, 2015	Nov. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Corporate Indebtedness
Increase in credit facility	$ 500,000,000.0
Outstanding under line of credit facility					$ 994,000,000	$ 385,000,000
Revolving credit facility
Corporate Indebtedness
Increase in credit facility	400,000,000.0
Credit facility, maximum borrowing capacity	1,600,000,000.0				1,600,000,000.0
Maximum borrowing capacity, optional expansion					$ 3,000,000,000.0
Effective rate (as a percent)					1.45%
Facility fee (as a percent)					0.25%
Outstanding under line of credit facility					$ 994,000,000
Revolving credit facility | Minimum
Corporate Indebtedness
Facility fee (as a percent)					0.125%
Revolving credit facility | Maximum
Corporate Indebtedness
Facility fee (as a percent)					0.30%
Revolving credit facility | LIBOR
Corporate Indebtedness
Interest rate added to base rate (as a percent)					1.25%
Revolving credit facility | LIBOR | Minimum
Corporate Indebtedness
Interest rate added to base rate (as a percent)					0.875%
Revolving credit facility | LIBOR | Maximum
Corporate Indebtedness
Interest rate added to base rate (as a percent)					1.55%
Term loan
Corporate Indebtedness
Increase in credit facility	100,000,000.0		$ 50,000,000.0	$ 383,000,000.0
Credit facility, maximum borrowing capacity	$ 933,000,000.0		$ 833,000,000	$ 783,000,000	$ 933,000,000
Decrease in basis points		0.20%		0.25%
Decrease in facility fee (as a percent)		0.05%
Effective rate (as a percent)					1.67%
Term loan | LIBOR
Corporate Indebtedness
Interest rate added to base rate (as a percent)					1.40%
Term loan | LIBOR | Minimum
Corporate Indebtedness
Interest rate added to base rate (as a percent)					0.95%
Term loan | LIBOR | Maximum
Corporate Indebtedness
Interest rate added to base rate (as a percent)					1.90%
2012 Credit Facility
Corporate Indebtedness
Letters of credit					$ 73,100,000
Ability to borrow under line of credit facility					$ 532,900,000