Stockholders' Equity (Narrative) (Details) - USD ($)		3 Months Ended	6 Months Ended
	Apr. 17, 2018	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Jul. 31, 2017	Jun. 30, 2017	Aug. 31, 2014
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		$ 10,357,600,000	$ 10,357,600,000	$ 10,216,200,000		$ 10,067,900,000
Preferred stock, without par value (in dollars per share)		$ 0	$ 0	$ 0
Stock repurchase, shares (in shares)		1,787,701	4,305,710
Stock repurchase, share-based compensation plan (in shares)		5,986	385,109
Stock repurchase, aggregate cost		$ 190,600,000	$ 453,800,000
Average price paid per share for common stock repurchased (in usd per share)		$ 106.63	$ 105.39
Stock repurchase program, number of shares authorized to be repurchased (in shares)					25,000,000
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount		$ 1,000,000,000	$ 1,000,000,000
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%
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%
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	Jul. 01, 2018
Dividends payable, date of record	Jun. 15, 2018