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Debt - Additional Information (Detail) - USD ($)				12 Months Ended
Debt - Additional Information (Detail) - USD ($)	Oct. 29, 2018	Sep. 27, 2018	Jun. 15, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 17, 2016
Debt Instrument [Line Items]
Debt, face amount				$ 1,667,487,000
Redemption of notes				6,875,000	$ 327,762,000	$ 126,875,000
Payments of additional capital to insurance subsidiaries	$ 150,000,000
Loss on redemption of debt				$ 0	$ 11,078,000	$ 4,041,000
Senior Notes, Due 2028 \| Senior notes
Debt Instrument [Line Items]
Debt, face amount		$ 550,000,000
Interest rate		4.55%
Proceeds from sale of notes		$ 543,000,000
Senior Notes Due October 2018 \| Senior notes
Debt Instrument [Line Items]
Interest rate	9.25%
Redemption of notes	$ 293,000,000
Payment of make-whole premium	11,000,000
Accrued interest	10,000,000
Senior Notes, Due 2019 \| Senior notes
Debt Instrument [Line Items]
Loss on redemption of debt	$ 11,000,000
Junior Subordinated Debentures Due 2036 \| Junior Subordinated Debt
Debt Instrument [Line Items]
Redemption of notes			$ 20,000,000
Term Loan Due 2021 \| Unsecured Debt
Debt Instrument [Line Items]
Debt, face amount							$ 100,000,000
Balloon payment							75,000,000
Term Loan Due 2021 \| Unsecured Debt \| LIBOR
Debt Instrument [Line Items]
Basis spread				1.25%
Term Loan Due 2021 \| Line of Credit \| Unsecured Debt
Debt Instrument [Line Items]
Maximum borrowing capacity							$ 750,000,000