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Benefit Plans - Additional Information (Detail) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jun. 08, 2016 | Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Share-based compensation expense | $ 3,198,000 | $ 3,091,000 | $ 3,399,000 |  |
Tax benefit related to share-based compensation expense | 1,300,000 | 1,200,000 | 1,300,000 |  |
Capitalized share-based compensation expense | $ 0 | $ 0 | $ 0 |  |
Stock option plans remain outstanding | 1,208,860 | 1,584,435 | 1,410,650 |  | 1,343,760
Weighted average grant date fair value | $ 6.28 | $ 4.14 | $ 6.60 |  |
Eligible period of service for participation in plan | 3 months |  |  |  |
Minimum age for employees to participate in KSOP | 21 years |  |  |  |
Dividends deducted |  | $ 300,000 |  |  |
Tax benefit from dividends | $ 100,000 |  |  |  |
Shares outstanding, employee stock ownership plan | 258,482 |  |  |  |
Percentage of total common shares outstanding | 2.00% |  |  |  |
Stock Options [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Number of additional shares authorized | 2,097,594 |  |  |  |
Stock options granted | 8,458,600 |  |  |  |
Stock options, exercise prices lower | $ 3.47 |  |  |  |
Stock options, exercise prices upper | $ 40.37 |  |  |  |
Purchase of shares | 5,620,878 |  |  |  |
Stock option shares terminated | 1,628,622 |  |  |  |
Stock option plans remain outstanding | 1,208,860 |  |  |  |
Stock options outstanding, exercise prices lower | $ 23.84 |  |  |  |
Stock options outstanding, exercise prices upper | $ 40.37 |  |  |  |
Stock options vesting period | 5 years |  |  |  |
Options Outstanding, Weighted-Average Remaining Contractual Life (Years) | 7 years |  |  |  |
Options issued to non-employee advisors | 0 |  |  |  |
Aggregate intrinsic value of options exercised and sold | $ 6,200,000 | $ 4,200,000 | $ 5,700,000 |  |
Total unrecognized compensation cost | $ 4,300,000 |  |  |  |
Expected recognition period of unrecognized compensation expense | 2 years 8 months 12 days |  |  |  |
Performance-based RSUs [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Stock options vesting period | 5 years |  |  |  |
Percentage of vesting annually at anniversary | 20.00% |  |  |  |
Number of RSU's expected to vest | 112,834 |  |  |  |
Performance-based RSUs [Member] | Vesting after three years [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Percentage of vesting RSUs | 60.00% |  |  |  |
Service-based RSUs [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Number of RSU's expected to vest | 17,600 |  |  |  |
Number of forfeitures expected | 0 |  |  |  |
Restricted Stock Units [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Expected recognition period of unrecognized compensation expense | 2 years 2 months 12 days |  |  |  |
Number of forfeitures expected | 66,200 | 68,300 | 80,320 |  |
Total fair value of RSU's vested based on weighted average grant date values | $ 1,000,000 | $ 1,800,000 | $ 2,500,000 |  |
Stock-based compensation expense for restricted stock | 1,400,000 | $ 1,000,000 | $ 1,700,000 |  |
Total unrecognized compensation expense net of forfeitures | $ 1,700,000 |  |  |  |
Maximum [Member] | Service-based RSUs [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Stock options vesting period | 14 months |  |  |  |
Minimum [Member] | Service-based RSUs [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Stock options vesting period | 12 months |  |  |  |
2016 Stock Incentive Plan [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Reduction in common stock available for grant | 2 |  |  |  |
Common Stock [Member] | 2016 Stock Incentive Plan [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Shares of common stock remained available for grants of awards |  |  |  | 2,000,000 |
Common Stock [Member] | 2016 Stock Incentive Plan [Member] | Maximum [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Term of award (in years) | 7 years |  |  |  |
Common Stock [Member] | 2016 Stock Incentive Plan [Member] | Minimum [Member] |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Rate of exercise price for equity award granted | 100.00% |  |  |  |

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- Definition Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=65895303&loc=d3e17540-113929 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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