Stockholders' Equity (Narrative) (Details) - USD ($)			3 Months Ended
	Jan. 21, 2020	Jul. 23, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Jul. 31, 2018
Class of Stock [Line Items]
Preferred stock, authorized (in shares)			10,000,000		10,000,000
Equity related to preferred stock			$ 10,879,700,000	$ 10,616,200,000	$ 11,091,000,000.0	$ 10,508,300,000
Preferred stock, without par value (in dollars per share)			$ 0		$ 0
Common stock dividends (in usd per share)			$ 0.70	$ 0.60
Stock repurchase, shares (in shares)			3,240,738
Stock repurchase, share-based compensation plan (in shares)			496,862
Stock repurchase, aggregate cost			$ 296,800,000	$ 257,400,000
Average price paid per share for common stock repurchased (in usd per share)			$ 91.59
Stock repurchase program, number of shares authorized to be repurchased (in shares)							25,000,000.0
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount			$ 531,700,000
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)			5,000		5,000
Per share ownership interest, percentage			1.00%
Equity related to preferred stock			$ 493,500,000		$ 493,500,000
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%
Preferred stock, dividends per share, declared (in dollars per share)		$ 2,300
Preferred stock, value, issued					493,500,000
Series D Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)			500,000
Shares issued (in shares)			500,000
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)			16,000
Per share ownership interest, percentage			0.10%
Equity related to preferred stock					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, value, issued			$ 391,400,000		$ 391,400,000
Common stock dividends (in usd per share)	$ 476.5278
Series E Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)			16,000,000
Shares issued (in shares)			16,000,000
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)			0		16,000
Preferred stock, redemption amount			$ 400,000,000
Preferred stock, redemption price per share(in dollars per share)			$ 25,000
Preferred stock, depositary share, redemption price per share (in dollars per share)			$ 25.00
Preferred stock redemption premium			$ 11,500,000
Shares issued (in shares)			16,000,000
Preferred stock, value, issued			$ 0		$ 388,500,000