Securities - Additional Information (Detail) |
3 Months Ended | ||
|---|---|---|---|
|
Mar. 31, 2018
USD ($)
Holdings
|
Mar. 31, 2017
USD ($)
|
Dec. 31, 2017
USD ($)
Holdings
|
|
| Investments, Debt and Equity Securities [Abstract] | |||
| Maximum percentage of equity of one issuer | 10.00% | ||
| Number of holdings greater than specified percentage of equity | Holdings | 0 | 0 | |
| Equities securities | $ 13,986,000 | $ 13,457,000 | |
| Securities with aggregate fair values | 1,500,000,000 | 1,400,000,000 | |
| Net unrealized losses on available-for-sale debt securities included in AOCI | 29,200,000 | 13,300,000 | |
| Sales of available-for-sale debt securities | 0 | $ 0 | |
| Impairment loss relating to securities | 0 | ||
| Federal home loan bank stock, Total | $ 55,600,000 | $ 45,900,000 | |