Shareholders' Equity (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended										12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shareholders' Equity
Common stock dividends declared (in dollars per share)		$ 0.48	$ 0.48	$ 0.375	$ 0.375	$ 0.375	$ 0.375	$ 0.3	$ 0.3	$ 0.30
Dividend declared prior quarter paid current quarter (in dollars per share)		$ 0.48			0.375	$ 0.375
Dividend declared and paid during the same period (in dollars per share)				$ 0.375	$ 0.375			$ 0.30	$ 0.30
Total cost of treasury stock, acquired during the period											$ 299,960,000	$ 200,000,000	$ 236,074,000
October 2015 Stock Repurchase
Shareholders' Equity
Stock repurchase program, authorized amount		$ 500,000,000				$ 500,000,000					$ 500,000,000	$ 500,000,000
Treasury stock, shares acquired					4.1	2.1
Total cost of treasury stock, acquired during the period					$ 300,000,000	$ 200,000,000
Subsequent event
Shareholders' Equity
Dividend declared prior quarter paid current quarter (in dollars per share)	$ 0.48