Stockholders' Equity (Narrative) (Details) - USD ($)			3 Months Ended	9 Months Ended
	Jul. 17, 2018	Apr. 17, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 18, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)			10,000,000	10,000,000	10,000,000
Equity related to preferred stock			$ 10,364,500,000	$ 10,364,500,000	$ 10,216,200,000	$ 10,165,200,000
Preferred stock, without par value (in dollars per share)			$ 0	$ 0	$ 0
Stock repurchase, shares (in shares)			2,171,699	6,477,409
Stock repurchase, share-based compensation plan (in shares)			54,672	439,781
Stock repurchase, aggregate cost			$ 235,900,000	$ 689,700,000
Average price paid per share for common stock repurchased (in usd per share)			$ 108.62	$ 106.48
Stock repurchase program, number of shares authorized to be repurchased (in shares)	25,000,000						9,500,000
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount			$ 764,100,000	$ 764,100,000
Series D Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Shares issued (in shares)			500,000	500,000
Preferred stock, liquidation preference			$ 100,000	$ 100,000
Depository shares, liquidation preference per share (in dollars per share)			$ 1,000	$ 1,000
Per share ownership interest, percentage			1.00%	1.00%
Series D Preferred Stock
Class of Stock [Line Items]
Equity related to preferred stock			$ 493,500,000	$ 493,500,000	$ 493,500,000
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
Series C Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Shares issued (in shares)			16,000,000	16,000,000
Preferred stock, liquidation preference			$ 25,000	$ 25,000
Depository shares, liquidation preference per share (in dollars per share)			$ 25	$ 25
Per share ownership interest, percentage			0.10%	0.10%
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock			$ 388,500,000.0	$ 388,500,000.0	388,500,000.0
Preferred stock, annual dividend rate				5.85%
Preferred stock, dividends per share, declared (in dollars per share)		$ 365.625
Dividends payable, date to be paid		Oct. 01, 2018
Dividends payable, date of record		Sep. 15, 2018
Preferred Stock
Class of Stock [Line Items]
Equity related to preferred stock					882,000,000			$ 882,000,000
Preferred Stock | Series D Preferred Stock
Class of Stock [Line Items]
Equity related to preferred stock			493,500,000	$ 493,500,000	$ 493,500,000	493,500,000		493,500,000
Preferred Stock | Series C Preferred Stock
Class of Stock [Line Items]
Equity related to preferred stock			$ 388,500,000	$ 388,500,000		$ 388,500,000		$ 388,500,000