Stockholders' Equity (Narrative) (Details) - USD ($)			3 Months Ended	6 Months Ended
	Apr. 25, 2017	Aug. 08, 2016	Jun. 30, 2017	Jun. 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
Preferred stock, without par value			$ 0	$ 0		$ 0
Stock repurchase, shares			1,760,264	2,571,994
Stock repurchase, share-based compensation plan, shares			4,342	415,211
Stock repurchase, aggregate cost			$ 157,600,000	$ 227,700,000
Average price paid per share for common stock repurchased			$ 89.52	$ 88.53
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount			$ 750,000,000	$ 750,000,000
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
Dividends payable, date to be paid	Jul. 01, 2017
Dividends payable, date of record	Jun. 15, 2017
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	$ 388,500,000		$ 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, annual dividend rate				5.85%
Preferred stock			$ 388,500,000	$ 388,500,000		$ 388,500,000
Preferred stock, dividends per share, declared	$ 365.625
Subsequent Event
Class of Stock [Line Items]
Stock repurchase authorized amount					$ 9,500,000