Stockholders' Equity (Narrative) (Details) - USD ($)			3 Months Ended	9 Months Ended
	Aug. 08, 2016	Jul. 20, 2016	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
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			951,401	5,231,246
Stock repurchase, share-based compensation plan, shares			42,012	429,855
Stock repurchase, aggregate cost			$ 64,900,000	$ 346,100,000
Average price paid per share for common stock repurchased			$ 68.26	$ 66.15
Stock repurchase authorized, remaining shares			4,472,756	4,472,756
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized shares			15,000,000	15,000,000
Stock repurchase authorized amount			$ 210,100,000	$ 210,100,000
Series D Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Issuance of Preferred Stock, Series D				493,500,000
Preferred stock, without par value	$ 0
Per share ownership interest, percentage	1.00%
Shares issued	500,000
Preferred Stock, Outstanding (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%
Series D Preferred Stock
Class of Stock [Line Items]
Proceeds from Issuance or Sale of Equity	$ 493,500,000
Preferred stock			$ 493,500,000	$ 493,500,000		$ 0
Preferred Stock, Outstanding (in shares)			5,000	5,000		0
Series C Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Preferred stock			$ 388,500,000	$ 388,500,000		$ 388,500,000
Preferred stock, without par value			$ 0	$ 0		$ 0
Per share ownership interest, percentage			0.10%	0.10%		0.10%
Preferred Stock, Outstanding (in shares)						16,000,000
Preferred stock, liquidation preference			$ 25,000	$ 25,000		$ 25,000
Depository shares, liquidation preference per share			$ 25	$ 25		$ 25
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock			$ 388,500,000	$ 388,500,000		$ 388,500,000
Preferred Stock, Outstanding (in shares)			16,000	16,000		16,000
Preferred stock, annual dividend rate				5.85%
Preferred stock, dividends per share, declared		$ 365.625
Dividends payable, date to be paid		Oct. 01, 2016
Dividends payable, date of record		Sep. 15, 2016
Preferred Stock
Class of Stock [Line Items]
Issuance of Preferred Stock, Series D				$ 493,500,000	$ 0