Investment in Securities - Bond by Credit Quality Rating Distribution (Detail) - Bonds - Credit Risk Financial Instruments - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
Concentration Risk [Line Items]
Amortized cost	$ 14,684,413		$ 14,473,070
Estimated fair value	$ 14,907,993		14,345,647
Percent of fair value	100.00%	100.00%
AAA
Concentration Risk [Line Items]
Amortized cost	$ 706,418		690,009
Estimated fair value	$ 725,774		702,531
Percent of fair value	4.90%	4.90%
AA
Concentration Risk [Line Items]
Amortized cost	$ 1,326,936		1,326,947
Estimated fair value	$ 1,358,217		1,336,380
Percent of fair value	9.10%	9.30%
A
Concentration Risk [Line Items]
Amortized cost	$ 5,460,570		5,350,316
Estimated fair value	$ 5,554,701		5,314,589
Percent of fair value	37.30%	37.00%
BBB
Concentration Risk [Line Items]
Amortized cost	$ 6,706,003		6,584,478
Estimated fair value	$ 6,798,672		6,507,212
Percent of fair value	45.60%	45.40%
BB and below
Concentration Risk [Line Items]
Amortized cost	$ 484,486		521,320
Estimated fair value	$ 470,629		$ 484,935
Percent of fair value	3.10%	3.40%