Stockholders' Equity (Narrative) (Details) - USD ($)		3 Months Ended				6 Months Ended
	Apr. 21, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Jul. 31, 2018
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)		10,000,000				10,000,000		10,000,000
Stockholders' equity		$ 11,382,200,000	$ 10,879,700,000	$ 10,805,500,000	$ 10,616,200,000	$ 11,382,200,000	$ 10,805,500,000	$ 11,091,000,000.0	$ 10,508,300,000
Common stock dividends (in usd per share)		$ 0.70	$ 0.70		$ 0.60		$ 0.60
Stock repurchase, share-based compensation plan (in shares)		3,349				500,211
Stock repurchase, aggregate cost		$ 200,000	$ 296,800,000	$ 271,200,000	$ 257,400,000	$ 297,000,000.0
Average price paid per share for common stock repurchased (in usd per share)		$ 79.19				$ 91.55
Stock repurchase, shares (in shares)						3,244,087
Stock repurchase program, number of shares authorized to be repurchased (in shares)										25,000,000.0
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		5,000				5,000		5,000
Stockholders' equity		$ 493,500,000				$ 493,500,000		$ 493,500,000
Preferred stock, liquidation preference		$ 100,000				$ 100,000
Depository shares, liquidation preference per share (in dollars per share)		$ 1,000				$ 1,000
Preferred stock, annual dividend rate						4.60%
Preferred stock, dividend payment rate, variable rate						3.202%
Preferred stock		$ 493,500,000				$ 493,500,000		$ 493,500,000
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		16,000				16,000		16,000
Percentage of ownership interest per each depository shares		0.10%				0.10%
Stockholders' equity		$ 391,400,000				$ 391,400,000		$ 391,400,000
Preferred stock, without par value (in dollars per share)		$ 0				$ 0
Preferred stock, liquidation preference		$ 25,000				$ 25,000
Depository shares, liquidation preference per share (in dollars per share)		$ 25				$ 25
Preferred stock, annual dividend rate						4.70%
Preferred stock		$ 391,400,000				$ 391,400,000		$ 391,400,000
Common stock dividends (in usd per share)	$ 293.75
Series D Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		500,000				500,000
Preferred stock, shares issued (in shares)		500,000				500,000
Percentage of ownership interest per each depository shares		1.00%				1.00%
Series E Preferred Stock Depositary Shares
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		16,000,000				16,000,000
Preferred stock, shares issued (in shares)		16,000,000				16,000,000