Recently Issued Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended								12 Months Ended
	Jan. 01, 2018	Apr. 30, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred policy acquisition costs											$ (51,920)	$ 2,083	$ 29,074
Net income from change in unrealized investment gains (losses) on securities											(136,261)	169,740	93,704
Net periodic pension costs net of tax			$ 3,133	$ (1,236)	$ (1,677)	$ (792)	$ (4,043)	$ (1,545)	$ (5,588)	$ (1,232)	572	12,408	$ 7,421
ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative unrealized gains and losses reclassified from accumulated other comprehensive income to retained earnings	$ 667,700
Offset by participating policyholder's interest	30,400
Deferred policy acquisition costs		$ 10,200
ASU 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic pension costs net of tax												$ 12,400
Investment Securities | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income from change in unrealized investment gains (losses) on securities											$ (84,700)
Retained Earnings | ASU 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings	$ 59,900