Stockholders' Equity (Narrative) (Details) - USD ($)			3 Months Ended	9 Months Ended
	Jul. 18, 2017	Aug. 08, 2016	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Apr. 21, 2015
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			$ 10,165,200,000	$ 10,165,200,000	$ 9,770,400,000	$ 9,579,800,000
Preferred stock, without par value			$ 0	$ 0	$ 0
Stock repurchase, shares			1,411,696	3,983,690
Stock repurchase, share-based compensation plan, shares			43,871	459,082
Stock repurchase, aggregate cost			$ 124,800,000	$ 352,500,000
Average price paid per share for common stock repurchased			$ 88.43	$ 88.50
Stock repurchase authorized amount	$ 9,500,000							$ 15,000,000
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount			$ 625,200,000	$ 625,200,000
Series D Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Shares issued (in shares)		500,000
Preferred stock, liquidation preference		$ 100,000
Depository shares, liquidation preference per share		$ 1,000
Preferred stock, annual dividend rate		4.60%
Preferred stock, dividend payment rate, variable rate		3.202%
Preferred stock, without par value		$ 0
Per share ownership interest, percentage		1.00%
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock		$ 493,500,000	$ 493,500,000	$ 493,500,000	$ 493,500,000
Preferred stock, dividends per share, declared	$ 2,300.00
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	$ 25,000
Depository shares, liquidation preference per share			$ 25	$ 25	$ 25
Preferred stock					$ 388,500,000.0
Preferred stock, without par value			$ 0	$ 0	$ 0
Per share ownership interest, percentage			0.10%	0.10%	0.10%
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock			$ 388,500,000	$ 388,500,000	$ 388,500,000
Preferred stock, annual dividend rate				5.85%
Preferred stock, dividends per share, declared	$ 365.625
Dividends payable, date to be paid	Oct. 01, 2017
Dividends payable, date of record	Sep. 15, 2017
Preferred Stock
Class of Stock [Line Items]
Equity related to preferred stock			882,000,000	$ 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	$ 388,500,000