Stockholders' Equity (Narrative) (Details) - USD ($)		3 Months Ended
	Jul. 21, 2015	Mar. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Preferred stock, shares authorized		10,000,000	10,000,000
Preferred stock		$ 388,500,000	$ 388,500,000
Preferred stock, without par value		$ 0	$ 0
Preferred Stock, Outstanding		16,000	16,000
Stock repurchase, shares		2,310,617
Stock repurchase, share-based compensation plan, shares		385,604
Average price paid per share for common stock repurchased		$ 60.71
Stock repurchase, aggregate cost		$ 140,300,000
Stock repurchase authorized, remaining shares		7,393,385
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized shares		15,000,000
Stock repurchase authorized amount		$ 145,000,000
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock		$ 388,500,000.0	$ 388,500,000.0
Stock issued during period		16,000,000
Shares issued		0	0
Preferred stock, without par value		$ 0	$ 0
Preferred Stock, Outstanding		16,000	16,000
Preferred stock, liquidation preference		$ 25,000	$ 25,000
Depository shares, liquidation preference per share		$ 25	$ 25
Preferred stock, annual dividend rate		5.85%
Preferred stock, dividends per share, declared	$ 365.625
Dividends payable, date to be paid	Apr. 01, 2016
Dividends payable, date of record	Mar. 15, 2016
Per share ownership interest, percentage		0.10%	0.10%