Schedule I - Financial Information of the Registrant (Parent Company) - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Merger termination fee	$ 50,000	$ 50,000
Dividends paid by guaranty	30,000		$ 25,000
Net unrealized investment gains	688,819	564,392
Parent company
Condensed Financial Statements, Captions [Line Items]
Merger termination fee	0	50,000	0
Expenses related to Mergers and strategic alternatives review		6,800	12,700
Executive insurance policy settlement expenses		2,200
Net unrealized investment gains	$ 688,819	564,392	514,248	$ 491,698
Parent company | Cumulative effect adjustments on adoption of new accounting standards (Note 1-H and Note 7)
Condensed Financial Statements, Captions [Line Items]
Net unrealized investment gains		$ 0	$ 0	$ 3,592