Revenue from Contracts with Customers - Summary of Disaggregation of Revenues from Contracts (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		$ 525,815	$ 504,902	$ 1,050,751	$ 999,890
Term Life Insurance Segment Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		328,233	296,868	655,979	593,712
Term Life Insurance Segment Revenues [Member] | Total Segment Revenues from Contracts with Customers [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		11,426	10,055	21,594	19,800
Term Life Insurance Segment Revenues [Member] | Total Segment Revenues from Contracts with Customers [Member] | Other, Net [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		11,426	10,055	21,594	19,800
Term Life Insurance Segment Revenues [Member] | Revenues from Sources Other Than Contracts with Customers (Segregated Funds) [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		316,807	286,813	634,385	573,912
Investment and Savings Products Segment Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		164,181	173,086	349,213	335,756
Investment and Savings Products Segment Revenues [Member] | Total Segment Revenues from Contracts with Customers [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		150,701	158,722	321,332	307,507
Investment and Savings Products Segment Revenues [Member] | Total Segment Revenues from Contracts with Customers [Member] | Other, Net [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		2,745	2,434	5,287	4,856
Investment and Savings Products Segment Revenues [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		62,812	71,438	143,704	138,435
Investment and Savings Products Segment Revenues [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		64,666	64,953	131,659	124,707
Investment and Savings Products Segment Revenues [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		20,478	19,897	40,682	39,509
Investment and Savings Products Segment Revenues [Member] | Revenues from Sources Other Than Contracts with Customers (Segregated Funds) [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		13,480	14,364	27,881	28,249
Corporate and Other Distributed Products Segment Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		33,401	34,948	45,559	70,422
Corporate and Other Distributed Products Segment Revenues [Member] | Total Segment Revenues from Contracts with Customers [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		11,217	9,152	19,751	17,275
Corporate and Other Distributed Products Segment Revenues [Member] | Total Segment Revenues from Contracts with Customers [Member] | Other, Net [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		865	1,335	1,820	2,392
Corporate and Other Distributed Products Segment Revenues [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]	10,352	7,817	17,931	14,883
Corporate and Other Distributed Products Segment Revenues [Member] | Revenues from Sources Other Than Contracts with Customers (Segregated Funds) [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		$ 22,184	$ 25,796	$ 25,808	$ 53,147

[1] Commissions and fees for the three and six months ended June 30, 2020 included $1.5 million and $2.6 million, respectively, and for the three and six months ended June 30, 2019 included $1.4 million and $2.5 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.