Investments - Schedule of Other Long-Term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Other long-term investments under fair value option	$ 315,535	$ 207,258
Commercial mortgage participations
Investment [Line Items]
Other long-term investments under fair value option	127,953	96,266
Other
Investment [Line Items]
Other long-term investments under fair value option	3,116	2,751
Partnership Interest - Fair Value Option \| Investment in limited partnerships
Investment [Line Items]
Other long-term investments under fair value option	$ 184,466	$ 108,241

Amount of long-term investments classified as other.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_OtherLongTermInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant