Segment and Geographical Information - Summary of Assets and Profit or Loss by Segment (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues		$ 467,815	$ 413,696	$ 927,737	$ 818,860
Net investment income		20,030	19,742	39,047	39,636
Amortization of deferred policy acquisition costs		53,847	47,861	114,011	99,710
Share-based compensation		1,891	1,923	13,392	11,667
Income (loss) before income taxes		113,765	96,389	196,727	171,230
Assets		12,496,641		12,496,641		$ 12,460,703
Term Life Insurance Segment [Member]
Segment Reporting Information [Line Items]
Revenues		272,978	238,901	543,286	472,953
Net investment income		3,246	2,347	6,334	4,650
Amortization of deferred policy acquisition costs		51,257	44,937	107,930	95,070
Share-based compensation		793	301	2,966	2,077
Income (loss) before income taxes		75,828	61,854	135,448	110,877
Assets		6,307,722		6,307,722		6,205,837
Investment and Savings Products Segment [Member]
Segment Reporting Information [Line Items]
Revenues		162,841	143,774	324,883	284,180
Amortization of deferred policy acquisition costs		2,080	2,310	5,521	4,044
Share-based compensation		325	361	1,518	1,541
Income (loss) before income taxes		43,227	39,684	83,214	76,803
Assets	[1]	2,512,547		2,512,547		2,684,717
Corporate and Other Distributed Products Segment [Member]
Segment Reporting Information [Line Items]
Revenues		31,996	31,021	59,568	61,727
Net investment income		16,784	17,395	32,713	34,986
Amortization of deferred policy acquisition costs		510	614	560	596
Share-based compensation		773	1,261	8,908	8,049
Income (loss) before income taxes		(5,290)	$ (5,149)	(21,935)	$ (16,450)
Assets		$ 3,676,372		$ 3,676,372		$ 3,570,149

[1] The Investment and Savings Products segment includes assets held in separate accounts. Excluding separate accounts, the Investment and Savings Products segment assets were $123.6 million and $112.0 million as of June 30, 2018 and December 31, 2017, respectively.