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Insurance Activity - Additional Information (Detail) - USD ($) $ in Millions |  | 9 Months Ended |
--- | --- | --- | ---
Aug. 31, 2009 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Effects of Reinsurance [Line Items] |  |  |  |
Quoted percentage to share base | 50.00% |  |  |
Balances in loss contract reserve |  | $ 0.0 |  | $ 5.6
Decreased value of loss contract reserve |  | $ 5.6 | $ 2.3 |
WMMRC [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Second loss layer risk percentage of range minimum | 5.00% |  |  |
Second loss layer risk percentage of range maximum | 10.00% |  |  |
First loss layer risk percentage of range minimum | 4.00% |  |  |
First loss layer risk percentage of range maximum | 5.00% |  |  |
Minimum period of reinsurance agreements |  | 5 years |  |
Maximum period of reinsurance agreements |  | 10 years |  |
WMMRC [Member] | Minimum [Member] | Credit Concentration Risk | Liabilities, Total [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Net of ceding commission, percentage | 25.00% |  |  |
WMMRC [Member] | Maximum [Member] | Credit Concentration Risk | Liabilities, Total [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Net of ceding commission, percentage | 40.00% |  |  |

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