Stockholders' Equity (Narrative) (Details) - USD ($)		3 Months Ended
	Jan. 23, 2018	Mar. 31, 2018	Dec. 31, 2017	Jul. 31, 2017	Mar. 31, 2017	Aug. 08, 2016	Aug. 31, 2014
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)		10,000,000	10,000,000
Equity related to preferred stock		$ 10,226,000,000	$ 10,216,200,000		$ 9,977,700,000
Preferred stock, without par value		$ 0	$ 0
Stock repurchase, shares (in shares)		2,518,409
Stock repurchase, share-based compensation plan (in shares)		379,123
Stock repurchase, aggregate cost		$ 263,200,000
Average price paid per share for common stock repurchased (in usd per share)		$ 104.51
Stock repurchase program, number of shares authorized to be repurchased (in shares)				9,500,000
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount		$ 191,400,000
Series D Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Shares issued (in shares)		500,000
Equity related to preferred stock		$ 493,500,000	$ 493,500,000
Preferred stock, without par value		$ 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%
Per share ownership interest, percentage						1.00%
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock		$ 493,500,000	493,500,000
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
Equity related to preferred stock		$ 388,500,000
Preferred stock, liquidation preference		$ 25,000
Depository shares, liquidation preference per share (in dollars per share)		$ 25
Preferred stock			388,500,000.0
Per share ownership interest, percentage							0.10%
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock		$ 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
Dividends payable, date to be paid	Apr. 01, 2018
Dividends payable, date of record	Mar. 15, 2018