Recently Issued Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended	9 Months Ended
	Jan. 01, 2018	Apr. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred policy acquisition costs			$ 82,452
Retained earnings			5,797,783			$ 5,413,952
Accumulated other comprehensive income (loss)			85,444			$ (99,738)
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 2016-01 | Investment Securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income from change in unrealized investment gains (losses) on securities			$ 222,800	$ 118,900
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset					$ 13,100
Operating lease liability					13,100
ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings					800
Accumulated other comprehensive income (loss)					$ (800)
Retained Earnings | ASU 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings	$ 59,900