Revenue from Contracts with Customers - Summary of Disaggregation of Revenues from Contracts (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		$ 654,686	$ 525,815	$ 1,292,399	$ 1,050,751
Term Life Insurance Segment [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		383,536	328,233	765,565	655,979
Term Life Insurance Segment [Member] | Total Segment Revenues from Contracts with Customers [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		12,313	11,426	24,125	21,594
Term Life Insurance Segment [Member] | Total Segment Revenues from Contracts with Customers [Member] | Other, Net [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		12,313	11,426	24,125	21,594
Term Life Insurance Segment [Member] | Revenues from Sources Other Than Contracts with Customers (Segregated Funds) [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		371,223	316,807	741,440	634,385
Investment and Savings Products Segment [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		238,012	164,181	461,434	349,213
Investment and Savings Products Segment [Member] | Total Segment Revenues from Contracts with Customers [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		221,810	150,701	429,604	321,332
Investment and Savings Products Segment [Member] | Total Segment Revenues from Contracts with Customers [Member] | Other, Net [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		2,958	2,745	5,907	5,287
Investment and Savings Products Segment [Member] | Total Segment Revenues from Contracts with Customers [Member] | Commissions and Fees [Member] | Sales-based Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		104,716	62,812	202,828	143,704
Investment and Savings Products Segment [Member] | Total Segment Revenues from Contracts with Customers [Member] | Commissions and Fees [Member] | Asset-based Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		92,288	64,666	177,901	131,659
Investment and Savings Products Segment [Member] | Total Segment Revenues from Contracts with Customers [Member] | Commissions and Fees [Member] | Account-based Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		21,848	20,478	42,968	40,682
Investment and Savings Products Segment [Member] | Revenues from Sources Other Than Contracts with Customers (Segregated Funds) [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		16,202	13,480	31,830	27,881
Corporate and Other Distributed Products Segment [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		33,138	33,401	65,400	45,559
Corporate and Other Distributed Products Segment [Member] | Total Segment Revenues from Contracts with Customers [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		16,676	11,217	31,081	19,751
Corporate and Other Distributed Products Segment [Member] | Total Segment Revenues from Contracts with Customers [Member] | Other, Net [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		1,042	865	1,875	1,820
Corporate and Other Distributed Products Segment [Member] | Total Segment Revenues from Contracts with Customers [Member] | Commissions and Fees [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers	[1]	15,634	10,352	29,206	17,931
Corporate and Other Distributed Products Segment [Member] | Revenues from Sources Other Than Contracts with Customers (Segregated Funds) [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		$ 16,462	$ 22,184	$ 34,319	$ 25,808

[1] Commissions and fees for the three and six months ended June 30, 2021 included $1.5 million and $2.6 million, respectively, and for the three and six months ended June 30, 2020 included $1.5 million and $2.6 million, respectively, attributable to performance obligations satisfied in a previous reporting period and represent the collection of variable consideration in the transaction price that had been previously constrained.