Corporate Indebtedness (Details) - USD ($)	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Corporate Indebtedness
Outstanding under line of credit facility	$ 195,125,000	$ 0
Long-term debt, carrying value	6,435,388,000
Revolving credit facility
Corporate Indebtedness
Credit facility, maximum borrowing capacity	1,600,000,000.0
Maximum borrowing capacity, optional expansion	$ 3,000,000,000.0
Effective rate (as a percent)	2.27%
Facility fee (as a percent)	0.25%
Outstanding under line of credit facility	$ 200,000,000
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
Credit facility, maximum borrowing capacity	$ 1,200,000,000.0
Effective rate (as a percent)	2.41%
Long-term debt, carrying value	$ 1,183,000,000
Term loan | Line of Credit
Corporate Indebtedness
Long-term debt, carrying value	$ 1,200,000,000	1,200,000,000
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	$ 80,800,000
Ability to borrow under line of credit facility	1,300,000,000
Long-term debt, carrying value	200,000,000
2012 Credit Facility | Line of Credit
Corporate Indebtedness
Long-term debt, carrying value	$ 195,100,000	$ (6,300,000)