Borrowing Arrangements (Scheduled Maturities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2016	$ 633,785
2017	774,958
2018	1,306,048
2019	1,916,915
2020	1,428,035
Thereafter	5,211,279
Total maturities	11,271,020	$ 10,888,295
Unrestricted cash and cash equivalents	53,023	55,348	$ 94,816	$ 67,908
Amount outstanding, net of cash and cash equivalents	$ 127,700
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	$ 602,661
2017	746,458
2018	1,101,879
2019	1,900,986
2020	1,416,913
Thereafter	5,085,663
Total maturities	10,854,560
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2016	0
2017	0
2018	180,683
2019	0
2020	0
Thereafter	0
Total maturities	180,683	919,099
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2016	31,124
2017	28,500
2018	23,486
2019	15,929
2020	11,122
Thereafter	125,616
Total maturities	235,777
Mortgage loans held for sale
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	22,900	$ 27,600
Senior Notes | 6.90% Senior Notes due 2037
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Thereafter	$ 52,400
Interest rate (as a percent)	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
Interest rate (as a percent)	6.59%