Investments - Net Investment Income (Parenthetical) (Details) - Deposit asset underlying 10% coinsurance agreement [Member] - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Net Investment Income, Reported Amounts, by Category [Line Items]
Losses recognized for change in fair value of deposit asset underlying the 10% coinsurance agreement	$ 400,000	$ 0	$ 1,700,000	$ 0
Percentage of coinsurance costs	10.00%	10.00%	10.00%	10.00%

Amount of realized and unrealized gain (loss) included in earnings, which have arisen from the use of significant unobservable inputs (level 3) to measure fair value of assets, liabilities, and financial instruments classified in shareholders' equity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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Balance Type:	credit
Period Type:	duration