Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	$ 182,752	$ 203,704
Real estate fund investments	354,804	462,132
Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	182,752	203,704
Real estate fund investments	354,804	462,132
Deferred compensation plan assets ($11,545 included in restricted cash and $97,633 in other assets)	109,178	121,374
Interest rate swaps (included in other assets)	27,472	21,816
Total assets	674,206	809,026
Mandatorily redeemable instruments (included in other liabilities)	520,561	50,561
Interest rate swaps (included in other liabilities)	1,052	10,122
Total liabilities	521,613	60,683
Recurring | Level 1
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	182,752	203,704
Real estate fund investments	0	0
Deferred compensation plan assets ($11,545 included in restricted cash and $97,633 in other assets)	69,050	63,930
Interest rate swaps (included in other assets)	0	0
Total assets	251,802	267,634
Mandatorily redeemable instruments (included in other liabilities)	520,561	50,561
Interest rate swaps (included in other liabilities)	0	0
Total liabilities	520,561	50,561
Recurring | Level 2
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	0	0
Real estate fund investments	0	0
Deferred compensation plan assets ($11,545 included in restricted cash and $97,633 in other assets)	0	0
Interest rate swaps (included in other assets)	27,472	21,816
Total assets	27,472	21,816
Mandatorily redeemable instruments (included in other liabilities)	0	0
Interest rate swaps (included in other liabilities)	1,052	10,122
Total liabilities	1,052	10,122
Recurring | Level 3
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable securities	0	0
Real estate fund investments	354,804	462,132
Deferred compensation plan assets ($11,545 included in restricted cash and $97,633 in other assets)	40,128	57,444
Interest rate swaps (included in other assets)	0	0
Total assets	394,932	519,576
Mandatorily redeemable instruments (included in other liabilities)	0	0
Interest rate swaps (included in other liabilities)	0	0
Total liabilities	0	0
Other Assets | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Deferred compensation plan assets ($11,545 included in restricted cash and $97,633 in other assets)	97,633	117,187
Restricted Cash | Recurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Deferred compensation plan assets ($11,545 included in restricted cash and $97,633 in other assets)	$ 11,545	$ 4,187