Net Investment Income and Realized Investment Gains (Losses) - Summary of Realized Investment Gains (Losses) and Other Than Temporary Impairment Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 2,947	$ 1,044
Other-than-temporary impairments	0	0
Debt Securities
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	2,602	667
Mortgage Loans
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	455	302
Real Estate
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	(158)	83
Other Invested Assets
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 48	$ (8)

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting.

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration