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	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred policy acquisition costs				$ 87,003	$ (51,920)	$ 2,083
Retained earnings				5,946,857	5,413,952
Accumulated other comprehensive income (loss)				99,518	(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 2016-13 | Minimum | Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative unrealized gains and losses reclassified from accumulated other comprehensive income to retained earnings			$ 30,000
ASU 2016-13 | Maximum | Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative unrealized gains and losses reclassified from accumulated other comprehensive income to retained earnings			$ 60,000
ASU 2017-07
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)
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				$ 333,800	$ (84,700)
Retained Earnings | ASU 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings	$ 59,900