Schedule II - Condensed Financial Information of the Registrant Cash Flow (Details) - USD ($) $ 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
Condensed Statements of Cash Flows
Net income	$ 28,102	$ 15,619	$ 25,934	$ 29,946	$ 18,346	$ 28,908	$ 26,816	$ 9,125	$ 99,601	$ 83,195	$ 62,387
Adjustments to reconcile net income to net cash provided by operating activities:
Earnings from consolidated subsidiaries									(1,552)	(1,182)	(1,016)
Changes in assets and liabilities:
Other assets									(1,864)	2,565	(5,400)
Accounts payable and accrued liabilities									3,395	10,678	(6,333)
Net cash provided by operating activities									112,456	127,691	82,040
Dividends paid									(52,667)	(48,813)	(45,460)
Net cash used for financing activities									(52,667)	(48,813)	(45,460)
Net increase (decrease) in cash and cash equivalents									6,825	(4,126)	21,656
Cash and cash equivalents at beginning of year				$ 37,582				$ 41,708	37,582	41,708	20,052
Cash and cash equivalents at end of period	$ 44,407				$ 37,582				44,407	37,582	41,708
Registrant
Condensed Statements of Cash Flows
Net income									99,601	83,195	62,387
Adjustments to reconcile net income to net cash provided by operating activities:
Earnings from consolidated subsidiaries									(101,295)	(84,890)	(63,881)
Dividends received from consolidated subsidiaries									47,585	45,271	41,826
Amortization of restricted stock expense									6,514	5,789	5,367
Changes in assets and liabilities:
Other assets									15	(60)	15
Accounts payable and accrued liabilities									247	(492)	(254)
Net cash provided by operating activities									52,667	48,813	45,460
Dividends paid									(52,667)	(48,813)	(45,460)
Net cash used for financing activities									$ (52,667)	$ (48,813)	$ (45,460)