Corporate Indebtedness (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Corporate Indebtedness
Outstanding under line of credit facility	$ 0	$ 985,055,000
Long-term Debt	6,568,712,000
Revolving credit facility
Corporate Indebtedness
Credit facility, maximum borrowing capacity	1,600,000,000.0
Maximum borrowing capacity, optional expansion	$ 3,000,000,000
Effective rate (as a percent)	1.78%
Facility fee (as a percent)	0.25%
Outstanding under line of credit facility	$ 0
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
Credit facility, maximum borrowing capacity	$ 1,183,000,000
Effective rate (as a percent)	2.27%
Long-term Debt	$ 1,183,000,000
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	$ 56,500,000
Ability to borrow under line of credit facility	1,500,000,000
Long-term Debt	0
Line of Credit [Member] | Term loan
Corporate Indebtedness
Long-term Debt	1,200,000,000	929,500,000
Line of Credit [Member] | 2012 Credit Facility
Corporate Indebtedness
Long-term Debt	$ (6,300,000)	$ 985,100,000