Revenue from Contracts with Customers - Summary of Disaggregation of Revenues from Contracts (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		$ 568,461	$ 520,653	$ 1,619,213	$ 1,520,545
Term Life Insurance Segment Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		357,799	314,527	1,013,778	908,239
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		12,717	10,985	34,311	30,785
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		12,717	10,985	34,311	30,785
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		345,082	303,542	979,467	877,454
Investment and Savings Products Segment Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		176,337	173,153	525,551	508,909
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		161,612	158,433	482,945	465,941
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		3,034	2,661	8,322	7,517
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		65,600	69,034	209,304	207,469
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		71,970	66,289	203,629	190,997
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		21,008	20,449	61,690	59,958
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		14,725	14,720	42,606	42,968
Corporate and Other Distributed Products Segment Revenues [Member]
Disaggregation Of Revenue [Line Items]
Disaggregation of revenues from contracts with customers		34,325	32,973	79,884	103,397
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		12,922	10,279	32,672	27,554
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		923	1,052	2,742	3,444
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]	11,999	9,227	29,930	24,110
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		$ 21,403	$ 22,694	$ 47,212	$ 75,843

[1] Commissions and fees for the three and nine months ended September 30, 2020 included $1.3 million and $3.9 million, respectively, and for the three and nine months ended September 30, 2019 included $1.7 million and $4.2 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.