Stockholders' Equity (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($) $ / shares shares		Sep. 30, 2016 USD ($) $ / shares		Jun. 30, 2016 USD ($) $ / shares		Mar. 31, 2016 USD ($) $ / shares		Dec. 31, 2016 USD ($) shares $ / shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) shares
Dividends, Common Stock, Cash | $	$ 344,500	[1]	$ 300,300	[1]	$ 300,800	[1]	$ 300,000	[1]	$ 1,244,423	$ 1,119,973
Conversion factor, Preferred Stock									1.188
4.5% Mandatory Convertible Preferred Stock, Par or Stated Value Per Share | $ / shares	$ 0.01								$ 0.01	$ 0.01
Purchases of common stock, value | $									$ 24,900	$ 29,700	$ 21,800
At the Market Stock Offering Program, aggregate value of common stock | $	$ 500,000								$ 500,000
Life to Date Shares Sold Under ATM	3,800,000								3,800,000
Life to Date Net Proceeds Under ATM | $	$ 323,800								$ 323,800
Life to Date Fees Under ATM | $	$ 3,300								3,300
Common Stock, Dividends, Per Share, Cash Paid | $ / shares	$ 0.95		$ 0.885		$ 0.885		$ 0.885
Preferred Stock, Dividends, Per Share, Cash Paid | $ / shares	$ 1.1250		$ 1.1250		$ 1.1250		$ 1.125
Dividends, Preferred Stock, Cash | $	$ 11,000		$ 11,000		$ 11,000		$ 11,000				$ 43,988
Preferred Stock [Member]
Dividends Payable, Date Declared	Sep. 17, 2016		Jun. 28, 2016		Mar. 22, 2016		Dec. 16, 2015
Dividends Payable, Date of Record	Oct. 15, 2016		Jul. 15, 2016		Apr. 15, 2016		Jan. 16, 2016
Dividends Payable, Date to be Paid	Nov. 01, 2016		Aug. 01, 2016		May 02, 2016		Feb. 01, 2016
Common Stock [Member]
Dividends Payable, Date Declared	Oct. 18, 2016		Aug. 02, 2016		May 20, 2016		Feb. 18, 2016
Dividends Payable, Date of Record	Dec. 16, 2016		Sep. 16, 2016		Jun. 17, 2016		Mar. 18, 2016
Dividends Payable, Date to be Paid	Dec. 30, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016
November 2016 Equity Offering [Member]
Proceeds from Issuance of Common Stock | $									$ 1,000,000
Preferred Stock [Member]
Conversion of Stock, Shares Converted									(9,775,000)
Common Stock [Member]
Stock Issued During Period, Shares, New Issues									15,178,064
Purchases of common stock, shares									(289,531)	(336,526)	(292,874)
Common Stock [Member] | November 2016 Equity Offering [Member]
Stock Issued During Period, Shares, New Issues									11,400,000
Preferred Stock [Member] | Preferred Stock [Member]
Conversion of Stock, Shares Converted									9,800,000
Preferred Stock [Member] | Common Stock [Member]
Conversion of Stock, Shares Issued									11,600,000

[1] Inclusive of dividends accrued for holders of unvested RSUs and payments of previously accrued dividends for holders of RSUs that have vested during the year ended December 31, 2016.