Borrowing Arrangements (Scheduled Maturities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2019	$ 406,629
2020	607,706
2021	844,493
2022	1,504,304
2023	1,551,398
Thereafter	5,915,172
Total maturities	10,829,702	$ 11,365,633
Unrestricted cash and cash equivalents	72,277	81,355
Line of credit facility, amount outstanding	$ 693,600
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
2019	$ 390,779
2020	592,384
2021	64,342
2022	1,491,561
2023	1,542,294
Thereafter	5,835,010
Total maturities	9,916,370
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2019	0
2020	0
2021	765,919
2022	0
2023	0
Thereafter	0
Total maturities	765,919	$ 535,832
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2019	15,850
2020	15,322
2021	14,232
2022	12,743
2023	9,104
Thereafter	80,162
Total maturities	147,413
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	6.90%
Senior Notes | 6.59% senior notes Due 2038
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Thereafter	$ 22,800
Interest rate	6.59%