Shareholders' Equity (Details) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended	3 Months Ended										12 Months Ended
	Feb. 28, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 30, 2017
Shareholders' Equity
Common stock dividends declared (in dollars per share)			$ 0.6	$ 0.6	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.375	$ 0.375	$ 0.375
Dividend declared and paid in same quarter (in dollars per share)						0.48			$ 0.375	0.375
Cash paid for dividends declared in prior period (in dollars per share)			$ 0.6	$ 0.48	$ 0.48	$ 0.48	$ 0.48			$ 0.48
Stock repurchase program, authorized amount															$ 500,000,000
Treasury stock, shares acquired				1.8
Total cost of treasury stock, acquired during the period				$ 225,000,000								$ 224,998,000	$ 299,960,000	$ 200,000,000
Remaining authorized repurchase amount			$ 275,000,000									275,000,000
Treasury stock, value			1,378,306,000				$ 1,153,308,000					1,378,306,000	$ 1,153,308,000
October 2015 Stock Repurchase
Shareholders' Equity
Stock repurchase program, authorized amount											$ 500,000,000			$ 500,000,000
Treasury stock, shares acquired											2.1		4.1
Total cost of treasury stock, acquired during the period											$ 200,000,000		$ 300,000,000
Treasury stock, value			$ 500,000,000									$ 500,000,000
Subsequent event
Shareholders' Equity
Cash paid for dividends declared in prior period (in dollars per share)		$ 0.6
Subsequent event | October 2015 Stock Repurchase
Shareholders' Equity
Treasury stock, shares acquired	1.5
Total cost of treasury stock, acquired during the period	$ 186,400,000