Schedule 1 - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows, Parent Company (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities
Net Cash Provided by Operating Activities	$ 185,037	$ 143,448	$ 173,577
Cash Flows from Investing Activities
Investment in Subsidiaries	(10,626)	(4,372)	(4,168)
Net Cash (Used in) Provided by Investing Activities	(209,472)	(107,419)	(132,590)
Cash Flows from Financing Activities
Change in Checks Written in Excess of Cash	(2,814)	(345)	2,434
Net Short-Term (Repayments) Borrowings	(12,599)	(93,772)	69,488
Proceeds from Issuance of Common Stock	20,338	0	4,349
Common Stock Issuance Expenses	(577)	(108)	0
Payments for Retirement of Capital Stock	(2,730)	(3,011)	(1,799)
Short-Term and Long-Term Debt Issuance Expenses	(950)	(761)	(380)
Payments for Retirement of Long-Term Debt	(172)	(189)	(48,231)
Dividends Paid	(55,723)	(53,198)	(50,632)
Net Cash Used in Financing Activities	44,773	(51,384)	(24,771)
Net Change in Cash and Cash Equivalents	20,338	(15,355)	16,216
Cash and Cash Equivalents at Beginning of Period	861	16,216	0
Cash and Cash Equivalents at End of Period	21,199	861	16,216
Parent Company [Member]
Cash Flows from Operating Activities
Net Cash Provided by Operating Activities	52,263	56,947	50,205
Cash Flows from Investing Activities
Investment in Subsidiaries	(34,990)	(24,764)	0
Debt Repaid by Subsidiaries	1,338	774	151
Cash Used in Investing Activities	(257)	(623)	(121)
Net Cash (Used in) Provided by Investing Activities	(33,909)	(24,613)	30
Cash Flows from Financing Activities
Change in Checks Written in Excess of Cash	(31)	31	0
Net Short-Term (Repayments) Borrowings	(3,215)	9,215	0
Borrowings from (Repayments to) Subsidiaries	28,985	(1,281)	23,389
Proceeds from Issuance of Common Stock	20,338	0	4,349
Common Stock Issuance Expenses	(577)	(108)	0
Payments for Retirement of Capital Stock	(2,730)	(3,011)	(1,799)
Short-Term and Long-Term Debt Issuance Expenses	(270)	(164)	(158)
Payments for Retirement of Long-Term Debt	(172)	(189)	(15,231)
Dividends Paid	(55,723)	(53,198)	(50,632)
Net Cash Used in Financing Activities	(13,395)	(48,705)	(40,082)
Net Change in Cash and Cash Equivalents	4,959	(16,371)	10,153
Cash and Cash Equivalents at Beginning of Period	0	16,371	6,218
Cash and Cash Equivalents at End of Period	$ 4,959	$ 0	$ 16,371