Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 08, 2017	Feb. 05, 2015
Stockholders Equity Note [Line Items]
Common stock authorized to be repurchased, number of shares				15,000,000.0	15,000,000.0
Common stock repurchased, value		$ 66,057	$ 123,193
Common stock dividends, paid per share	$ 0.15	$ 0.12	$ 0.11
Common stock dividends, total cash paid	$ 82,605	$ 65,920	$ 61,399
Payments of Ordinary Dividends, Common Stock	82,605	$ 65,920	$ 61,399
Common Stock and Additional Paid-in Capital
Stockholders Equity Note [Line Items]
Common stock repurchased, shares		2,132,000	4,169,000
Common stock repurchased, value		$ 24,222	$ 44,387
Retained Earnings
Stockholders Equity Note [Line Items]
Common stock repurchased, value		41,835	78,806
Common stock dividends, total cash paid	$ 82,605	$ 65,920	$ 61,399