Stockholders' Equity - Narrative (Details) - USD ($)		3 Months Ended			6 Months Ended
	May 15, 2017	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Oct. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Payments of dividends					$ 34,879,000	$ 34,029,000
Dividends declared per common share (USD per share)	$ 0.97	$ 0.97	$ 0.97	$ 0.92	$ 1.94	$ 1.84
Dividends payable		$ 17,490,000	$ 17,465,000		$ 17,490,000
Treasury stock reissued (shares)					235,152
Treasury stock reissued					$ 8,400,000
October 2015 Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount							$ 150,000,000