Stockholders' Equity (Narrative) (Details) - USD ($)		3 Months Ended
	Jan. 20, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Jul. 31, 2018
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)		10,000,000		10,000,000
Stockholders' equity		$ 11,457,900,000	$ 10,879,700,000	$ 11,688,300,000	$ 11,091,000,000.0
Common stock dividends (in usd per share)		$ 0.70	$ 0.70
Stock repurchase program, number of shares authorized to be repurchased (in shares)						25,000,000.0
Stock repurchase, shares (in shares)		1,399,673
Stock repurchase, share-based compensation plan (in shares)		365,697
Stock repurchase, aggregate cost		$ 135,600,000	$ 296,800,000
Average price paid per share for common stock repurchased (in usd per share)		$ 96.91
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		5,000		5,000
Stockholders' equity		$ 493,500,000		$ 493,500,000
Preferred stock, without par value (in dollars per share)		$ 0
Preferred stock, liquidation preference		$ 100,000
Depository shares, liquidation preference per share (in dollars per share)		$ 1,000
Preferred stock, annual dividend rate		4.60%
Preferred stock, dividend payment rate, variable rate		3.202%
Common stock dividends (in usd per share)	$ 2,300
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		16,000		16,000
Percentage of ownership interest per each depository shares		0.10%
Stockholders' equity		$ 391,400,000		$ 391,400,000
Preferred stock, without par value (in dollars per share)		$ 0
Preferred stock, liquidation preference		$ 25,000
Depository shares, liquidation preference per share (in dollars per share)		$ 25
Preferred stock, annual dividend rate		4.70%
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
Preferred stock, shares issued (in shares)		500,000
Percentage of ownership interest per each depository shares		1.00%
Series E Preferred Stock Depositary Shares
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		16,000,000
Preferred stock, shares issued (in shares)		16,000,000