Schedule I - Financial Information of the Registrant (Parent Company) - Additional Information (Details) - USD ($) $ in Thousands			12 Months Ended
	Apr. 27, 2016	Jan. 26, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Jan. 01, 2016
Condensed Financial Statements, Captions [Line Items]
Dividends paid by guaranty			$ 25,000	$ 20,000	$ 20,000
Net unrealized investment gains			(2,162)	0	0
Additional cash issued to class B common stock holders			0	0	12,000
Unrecognized excess tax benefits from additional paid-in capital to retained earnings						$ 0	$ 0
Total income tax benefit resulting from the 2017 Act			800	7,800
Parent Company
Condensed Financial Statements, Captions [Line Items]
Additional cash issued to class B common stock holders			0	0	12,000
Unrecognized excess tax benefits from additional paid-in capital to retained earnings			3,592	0	631
Total income tax benefit resulting from the 2017 Act				1,200
Parent Company | Other Operating Expense
Condensed Financial Statements, Captions [Line Items]
Expenses related to Mergers and strategic alternatives review			12,700	$ 2,900
Common stock exchange costs					3,400
Conversion, Class B Common Stock to Common Stock
Condensed Financial Statements, Captions [Line Items]
Additional cash issued to class B common stock holders	$ 12,000	$ 12,000	12,000		$ 12,000
Retained Earnings
Condensed Financial Statements, Captions [Line Items]
Unrecognized excess tax benefits from additional paid-in capital to retained earnings						$ 3,592	$ 631
ASU 2016-01 | Retained Earnings
Condensed Financial Statements, Captions [Line Items]
Net unrealized investment gains			$ 4,600