Significant Accounting Policies Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies [Abstract]
Net income	$ 38,605	$ 7,916	$ 14,616	$ 14,036	$ 13,114	$ 12,527	$ 16,368	$ 14,377	$ 75,173	$ 56,386	$ 73,056
Weighted Average Number of Shares Outstanding, Basic									83,298	83,297	86,974
Basic EPS	$ 0.45	$ 0.10	$ 0.18	$ 0.17	$ 0.16	$ 0.15	$ 0.20	$ 0.17	$ 0.90	$ 0.68	$ 0.84
Dilutive Securities, Effect on Basic Earnings Per Share, Including Options and Restrictive Stock Units									$ 0	$ 0	$ 0
Incremental Common Shares Attributable to Share-based Payment Arrangements									38	68	135
Net Income (Loss) Available to Common Stockholders, Diluted									$ 75,173	$ 56,386	$ 73,056
Weighted Average Number of Shares Outstanding, Diluted									83,336	83,365	87,109
Diluted EPS	$ 0.45	$ 0.09	$ 0.18	$ 0.17	$ 0.16	$ 0.15	$ 0.20	$ 0.17	$ 0.90	$ 0.68	$ 0.84