Stockholders' Equity (Narrative) (Details) - USD ($)		3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 21, 2015	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Stockholders Equity Note [Line Items]
Preferred stock, shares authorized		10,000,000	10,000,000	10,000,000
Preferred stock		$ 388,500,000	$ 388,500,000	$ 388,500,000
Stock repurchase, shares		1,914,950	4,766,083
Average price paid per share for common stock repurchased		$ 73.37	$ 72.27
Stock repurchase, aggregate cost		$ 140,500,000	$ 344,400,000
Stock repurchase authorized, remaining shares		11,789,787	11,789,787
Preferred stock, without par value		$ 0	$ 0	$ 0
Upper Limit
Stockholders Equity Note [Line Items]
Stock repurchase authorized shares		15,000,000	15,000,000
Stock repurchase authorized amount		$ 437,807,914	$ 437,807,914
Series C Preferred Stock
Stockholders Equity Note [Line Items]
Preferred stock				$ 388,500,000
Stock issued during period			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, annual dividend rate			5.85%
Preferred stock, dividends per share, declared	$ 365.625
Dividends payable, date to be paid	Oct. 01, 2015
Dividends payable, date of record	Sep. 15, 2015
Preferred stock, without par value				$ 0
Per share ownership interest, percentage				0.10%