Stockholders' Equity - Narrative (Details) - USD ($)		3 Months Ended			9 Months Ended
	May 15, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Oct. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Payments of dividends					$ 52,326,000	$ 50,790,000
Dividends declared per common share (USD per share)	$ 0.97	$ 0.97	$ 0.97	$ 0.92	$ 2.91	$ 2.76
Dividends payable		$ 17,755,000	$ 17,465,000		$ 17,755,000
Treasury stock reissued (shares)					273,376
Treasury stock reissued					$ 9,800,000
October 2015 Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount							$ 150,000,000