Stockholders' Equity (Narrative) (Details) - USD ($)			3 Months Ended						9 Months Ended
	Jul. 23, 2019	Apr. 23, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Preferred stock, authorized (in shares)			10,000,000						10,000,000	10,000,000
Preferred stock, without par value (in dollars per share)			$ 0						$ 0	$ 0
Equity related to preferred stock			$ 10,811,300,000	$ 10,805,500,000	$ 10,616,200,000	$ 10,364,500,000	$ 10,357,600,000	$ 10,226,000,000.0	$ 10,811,300,000	$ 10,508,300,000		$ 10,216,300,000
Stock repurchase, shares (in shares)			3,392,062						9,169,744
Stock repurchase, share-based compensation plan (in shares)			68,347						602,542
Stock repurchase, aggregate cost			$ 307,200,000	271,200,000	257,400,000	235,900,000	190,600,000	263,200,000	$ 835,800,000
Average price paid per share for common stock repurchased (in usd per share)			$ 90.55						$ 91.15
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			$ 1,100,000,000						$ 1,100,000,000
Series D Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Shares issued (in shares)			500,000						500,000
Preferred stock, outstanding (in shares)			500,000						500,000
Per share ownership interest, percentage			1.00%						1.00%
Preferred stock, without par value (in dollars per share)			$ 0						$ 0
Preferred stock, liquidation preference			$ 100,000						$ 100,000
Depository shares, liquidation preference per share (in dollars per share)			$ 1,000						$ 1,000
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)			5,000						5,000	5,000
Preferred stock			$ 493,500,000						$ 493,500,000	$ 493,500,000
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
Equity related to preferred stock										$ 493,500,000
Series C Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Shares issued (in shares)			16,000,000						16,000,000
Preferred stock, outstanding (in shares)			16,000,000						16,000,000
Per share ownership interest, percentage			0.10%						0.10%
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
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)			16,000						16,000	16,000
Preferred stock			$ 388,500,000						$ 388,500,000	$ 388,500,000
Preferred stock, annual dividend rate									5.85%
Preferred stock, dividends per share, declared (in dollars per share)		$ 365.625
Preferred Stock
Class of Stock [Line Items]
Equity related to preferred stock			882,000,000.0	$ 882,000,000.0	$ 882,000,000.0	$ 882,000,000.0	$ 882,000,000.0	$ 882,000,000.0	$ 882,000,000.0	$ 882,000,000.0		$ 882,000,000.0
Preferred Stock | Series C Preferred Stock
Class of Stock [Line Items]
Equity related to preferred stock			$ 388,500,000						$ 388,500,000