SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF PARENT 2 (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	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
Condensed financial statements
Income before income tax expense									$ 81,888	$ 108,341	$ 94,478
Net Income	$ 13,789	$ 22,952	$ 16,348	$ 10,782	$ 12,868	$ 21,006	$ 22,792	$ 12,701	$ 63,871	$ 69,367	$ 59,743
Weighted Average Number of Shares Outstanding (in shares)									36,733	36,638	36,552
Earnings Per Share, Basic	$ 0.37	$ 0.62	$ 0.44	$ 0.29	$ 0.35	$ 0.57	$ 0.62	$ 0.35	$ 1.73	$ 1.88	$ 1.63
Weighted Average Number of Diluted Common Shares Outstanding									36,936	36,844	36,750
Earnings Per Share, Diluted	$ 0.37	$ 0.62	$ 0.44	$ 0.29	$ 0.35	$ 0.57	$ 0.62	$ 0.34	$ 1.72	$ 1.88	$ 1.62
Dividends Declared Per Common Share (in dollars per share)									$ 1.06	$ 0.994	$ 0.914
Parent Company
Condensed financial statements
Operating revenues and other income									$ 0	$ 0	$ 71
Operating Expenses and Other Expenses									305	344	19
Operating income / (loss)									(305)	(344)	52
Equity in earnings of subsidiaries									63,651	67,490	59,145
Income before income tax expense									63,346	67,146	59,197
Income tax expense (benefit)									(525)	(2,221)	(546)
Net Income									$ 63,871	$ 69,367	$ 59,743
Earnings Per Share, Basic									$ 1.73	$ 1.88	$ 1.63
Earnings Per Share, Diluted									1.72	1.88	1.62
Dividends Declared Per Common Share (in dollars per share)									$ 1.060	$ 0.994	$ 0.914