Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Cash Dividend on Common Stock	$ 0.85	$ 0.85	$ 0.85	$ 0.80	$ 2.55	$ 2.40
Aggregated Dividend Paid	$ 43.6	$ 43.6	$ 43.6
Dividend payable date declared	Sep. 17, 2018	Jun. 18, 2018	Feb. 23, 2018
Common stock Dividend Payable Date	Oct. 15, 2018	Jul. 16, 2018	Apr. 16, 2018
Dividend payable date of record	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018