Quarterly Financial Information (Unaudited) (Summarized Unaudited Consolidated Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 923,263	$ 936,538	$ 942,304	$ 943,705	$ 895,288	$ 899,928	$ 895,490	$ 883,443	$ 3,745,810	$ 3,574,149	$ 3,443,522
Income from continuing operations	63,302	103,281	169,300	80,108	393,820	615,210	152,991	199,201	415,991	1,361,222	652,412
Discontinued operations	0	0	0	(10)	(15)	(19)	(23)	(53)	(10)	(110)	(922)
Net income	63,302	103,281	169,300	80,098	393,805	615,191	152,968	199,148	415,981	1,361,112	651,490
Comprehensive income attributable to noncontrolling interests	1,029	1,309	2,781	1,395	1,251	1,233	1,137	1,021	6,514	4,642	2,259
Net income attributable to common stockholders	$ 62,273	$ 101,972	$ 166,519	$ 78,703	$ 392,554	$ 613,958	$ 151,831	$ 198,127	$ 409,467	$ 1,356,470	$ 649,231
Basic:
Income from continuing operations (USD per share)	$ 0.18	$ 0.29	$ 0.48	$ 0.22	$ 1.11	$ 1.73	$ 0.43	$ 0.56	$ 1.17	$ 3.83	$ 1.89
Net income attributable to common stockholders (USD per share)	0.17	0.29	0.47	0.22	1.10	1.72	0.43	0.56	1.15	3.82	1.88
Diluted:
Income from continuing operations (USD per share)	0.18	0.29	0.47	0.22	1.10	1.71	0.43	0.56	1.16	3.80	1.87
Net income attributable to common stockholders (USD per share)	0.17	0.28	0.46	0.22	1.09	1.71	0.42	0.55	1.14	3.78	1.86
Dividends declared per common share (in dollars per share)		$ 0.79	$ 0.79	$ 0.79	$ 0.79	$ 0.775	$ 0.775	$ 0.775	$ 3.1625	3.115	2.965
Dividend declared in current year and taxable in following year (in dollars per share)	$ 0.79250									$ 0.79000	$ 0