Borrowing Arrangements (Scheduled Maturities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Feb. 29, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2016	$ 640,408,000
2017	1,269,502,000
2019	1,708,247,000
2020	1,428,533,000
2021	784,445,000
Thereafter	5,359,779,000
Total maturities	11,190,914,000		$ 11,271,020,000
Unrestricted cash and cash equivalents	286,707,000		53,023,000	$ 55,348,000	$ 94,816,000
Amount outstanding, net of cash and cash equivalents	$ (140,200,000)
Interest rate		1.132%
Minimum percentage of total unencumbered assets as a percentage of unsecured debt required to be maintained (as a percent)	150.00%
Principal Amount Due at Maturity
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2016	$ 614,438,000
2017	1,101,879,000
2019	1,693,640,000
2020	1,416,913,000
2021	774,318,000
Thereafter	5,242,559,000
Total maturities	10,843,747,000
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2016	0
2017	146,538,000
2019	0
2020	0
2021	0
Thereafter	0
Total maturities	146,538,000		180,683,000
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2016	25,970,000
2017	21,085,000
2019	14,607,000
2020	11,620,000
2021	10,127,000
Thereafter	117,220,000
Total maturities	200,629,000
Mortgage loans held for sale
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	0		$ 22,900,000
Senior Notes | 6.90% Senior Notes due 2037
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Thereafter	$ 52,400,000
Interest rate	6.90%
Senior Notes | 6.59% senior notes due 2038
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Thereafter	$ 23,000,000
Interest rate	6.59%