SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF PARENT 2 (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020	Jun. 30, 2020	Feb. 11, 2020
Condensed financial statements
Line of Credit Facility, Maximum Borrowing Capacity	$ 225,000,000.0								$ 225,000,000.0
Income tax expense									24,670,000	$ 18,017,000	$ 38,974,000
Net Income	$ 16,700,000	$ 28,006,000	$ 26,784,000	$ 12,852,000	$ 13,789,000	$ 22,952,000	$ 16,348,000	$ 10,782,000	$ 84,342,000	$ 63,871,000	$ 69,367,000
Weighted Average Number of Shares Outstanding (in shares)									36,814	36,733	36,638
Earnings Per Share, Basic	$ 0.45	$ 0.76	$ 0.72	$ 0.35	$ 0.37	$ 0.62	$ 0.44	$ 0.29	$ 2.28	$ 1.73	$ 1.88
Weighted Average Number of Diluted Common Shares Outstanding									36,964	36,936	36,844
Earnings Per Share, Diluted	$ 0.45	$ 0.76	$ 0.72	$ 0.35	$ 0.37	$ 0.62	$ 0.44	$ 0.29	$ 2.28	$ 1.72	$ 1.88
Dividends Declared Per Common Share (in dollars per share)									$ 1.160	$ 1.060	$ 0.994
Parent Company
Condensed financial statements
Line of Credit Facility, Maximum Borrowing Capacity	$ 225,000,000.0								$ 225,000,000.0
Operating revenues and other income									0	$ 0	$ 0
Operating Expenses and Other Expenses									314,000	305,000	344,000
Operating income / (loss)									(314,000)	(305,000)	(344,000)
Equity in earnings of subsidiaries									83,947,000	63,651,000	67,490,000
Closed Block Operations, Results before Income Taxes									83,633,000	63,346,000	67,146,000
Income tax expense									(709,000)	(525,000)	(2,221,000)
Income tax expense (benefit)										(525,000)	(2,221,000)
Net Income									$ 84,342,000	$ 63,871,000	$ 69,367,000
Earnings Per Share, Basic									$ 2.28	$ 1.73	$ 1.88
Earnings Per Share, Diluted									2.28	1.72	1.88
Dividends Declared Per Common Share (in dollars per share)									$ 1.160	$ 1.060	$ 0.994
Subsequent Event [Member] | Parent Company
Condensed financial statements
Line of Credit Facility, Maximum Borrowing Capacity												$ 200,000,000.0	$ 200,000,000.0	$ 260,000,000.0