Investment in Securities - Bond by Credit Quality Rating Distribution (Detail) - Bonds - Credit Risk Financial Instruments - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Amortized cost	$ 15,003,514	$ 14,473,070
Estimated fair value	$ 15,511,516	$ 14,345,647
Percent of fair value	100.00%	100.00%
AAA
Concentration Risk [Line Items]
Amortized cost	$ 771,497	$ 690,009
Estimated fair value	$ 798,910	$ 702,531
Percent of fair value	5.20%	4.90%
AA
Concentration Risk [Line Items]
Amortized cost	$ 1,302,861	$ 1,326,947
Estimated fair value	$ 1,348,881	$ 1,336,380
Percent of fair value	8.70%	9.30%
A
Concentration Risk [Line Items]
Amortized cost	$ 5,597,573	$ 5,350,316
Estimated fair value	$ 5,803,852	$ 5,314,589
Percent of fair value	37.40%	37.00%
BBB
Concentration Risk [Line Items]
Amortized cost	$ 6,873,228	$ 6,584,478
Estimated fair value	$ 7,109,627	$ 6,507,212
Percent of fair value	45.80%	45.40%
BB and below
Concentration Risk [Line Items]
Amortized cost	$ 458,355	$ 521,320
Estimated fair value	$ 450,246	$ 484,935
Percent of fair value	2.90%	3.40%