Stockholders' Equity (Narrative) (Details) - USD ($)				3 Months Ended
	Jan. 18, 2017	Jan. 17, 2017	Aug. 08, 2016	Mar. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)				10,000,000	10,000,000
Preferred stock, without par value				$ 0	$ 0
Stock repurchase, shares				811,730
Stock repurchase, share-based compensation plan, shares				410,869
Stock repurchase, aggregate cost				$ 70,100,000
Average price paid per share for common stock repurchased				$ 86.39
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount				$ 74,800,000
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, dividends, per share, cash paid		$ 2,977.222222
Preferred stock			$ 493,500,000	$ 493,500,000
Dividends payable, date to be paid	Apr. 01, 2017
Dividends payable, date of record	Mar. 15, 2017
Series C Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Shares issued (in shares)				16,000,000
Preferred stock, liquidation preference				$ 25,000	$ 25,000
Depository shares, liquidation preference per share				$ 25	$ 25
Preferred stock				$ 388,500,000	$ 388,500,000.0
Preferred stock, without par value				$ 0	$ 0
Per share ownership interest, percentage				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
Preferred stock, dividends per share, declared	$ 365.625