Subsequent Events Subsequent Events (Details Textual) - USD ($)		3 Months Ended								12 Months Ended
	Feb. 01, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 14, 2018	Nov. 15, 2017	Oct. 17, 2016
Subsequent Event [Line Items]
Stock Repurchase Program, Authorized Amount													$ 350,000,000	$ 150,000,000	$ 400,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 110,100,000								$ 110,100,000
Dividends declared per share (usd per share)		$ 0.60	$ 0.60	$ 0.60	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 2.30	$ 2.00	$ 2.00
Dividend Declared [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends declared per share (usd per share)	$ 0.60