Stockholders' Equity (Narrative) (Details) - USD ($)				3 Months Ended	6 Months Ended
	Apr. 01, 2016	Mar. 15, 2016	Jan. 19, 2016	Jun. 30, 2016	Jun. 30, 2016	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				$ 388,500,000	$ 388,500,000	$ 388,500,000
Preferred stock, without par value				$ 0	$ 0	$ 0
Preferred Stock, Outstanding (in shares)				16,000	16,000	16,000
Stock repurchase, shares				1,969,228	4,279,845
Stock repurchase, share-based compensation plan, shares				2,239	387,843
Stock repurchase, aggregate cost				$ 140,800,000	$ 281,100,000
Average price paid per share for common stock repurchased				$ 71.52	$ 65.68
Stock repurchase authorized, remaining shares				5,424,157	5,424,157
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized shares				15,000,000	15,000,000
Stock repurchase authorized amount				$ 275,000,000	$ 275,000,000
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock				$ 388,500,000	$ 388,500,000	$ 388,500,000
Stock issued during period					16,000,000
Preferred stock, without par value				$ 0	$ 0	$ 0
Per share ownership interest, percentage				0.10%	0.10%	0.10%
Shares issued						16,000,000
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
Preferred stock, annual dividend rate					5.85%
Preferred stock, dividends per share, declared			$ 365.625
Dividends payable, date to be paid	Jul. 01, 2016
Dividends payable, date of record		Jun. 15, 2016