Decrease in share based compensation expense.

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Percentage of Stock Award Subject to Vesting

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Share based compensation arrangement by share based payment award, award vesting date.

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Share based compensation arrangement by share based payment other than options expiration date.

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Stock option plan expense anticipated cost.

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Amount of increase in additional paid in capital (APIC) resulting from a tax benefit associated with share-based compensation plan other than an employee stock ownership plan (ESOP). Includes, but is not limited to, excess tax benefit.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Price of a single share of a number of saleable stocks of a company.

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Percentage of vesting of share-based compensation awards.

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The noncash expense that accounts for the value of stock or unit options distributed to employees as compensation.

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Share-Based Compensation Plans - Additional Information (Detail) - USD ($)		12 Months Ended				156 Months Ended	192 Months Ended
	Mar. 06, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options awarded		300,000	0	0	300,000
Exercise price applicable		$ 35.06	$ 25.13	$ 25.13		$ 25.94	$ 35.06
Strike price per share		$ 50.00
Compensation expense, options		$ 300,000	$ (400,000)	$ 300,000
Proceed from exercise of options		$ 0	0	0
Fair value of share		$ 36.23					$ 36.23
Weighted average fair value of options granted		$ 3.79
Compensation expense, restricted stock		$ 3,100,000	$ 4,100,000	$ 3,200,000
Stock options forfeited		100,000		200,000
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted		70,424	49,624	156,531		1,423,478	1,700,057
Unrecognized compensation expense for non-vested restricted stock		$ 1,300,000					$ 1,300,000
Weighted average life of non-vested restricted stock		1 year 8 months 12 days
Weighted average fair value per share		$ 38.85	$ 39.42	$ 29.44
Shares vested		166,117	116,111	111,205
Restricted Stock \| Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted		38,778	22,503	121,346		945,269	1,127,896
Weighted average fair value per share		$ 40.57	$ 38.21	$ 28.97
Shares vested		11,843
Restricted Stock \| Key Employees \| Share-based Compensation Award, Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date		16.50%	16.50%	16.50%
Vesting date		Jan. 01, 2019	Jan. 01, 2018	Jan. 01, 2017
Restricted Stock \| Key Employees \| Share-based Compensation Award, Tranche 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date		16.50%	16.50%	16.50%
Vesting date		Jan. 01, 2020	Jan. 01, 2019	Jan. 01, 2018
Restricted Stock \| Key Employees \| Share-based Compensation Award, Tranche 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date		17.00%	17.00%	17.00%
Vesting date		Jan. 01, 2021	Jan. 01, 2020	Jan. 01, 2019
Restricted Stock \| Key Employees \| Time Based Vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted				96,585
Restricted Stock \| Key Employees \| Vesting Schedule Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date		100.00%	100.00%	100.00%
Vesting date		Mar. 15, 2021	Mar. 15, 2020	Mar. 15, 2019
Percentage of stock award subject to vesting		50.00%	50.00%	50.00%
Restricted Stock \| Chief Executive Officer \| Each subsequent anniversary date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted				11,199
Percentage of shares vested on each anniversary of the grant date				33.33%
Vesting period				3 years
Restricted Stock \| Another Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted				5,309
Vesting date				Feb. 07, 2019
Restricted Stock \| Another Key Employees \| Vest on February 7, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date				100.00%
Restricted Stock \| Other Key Employees \| Personal Lines
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted				8,253
Restricted Stock \| Other Key Employees \| Share-based Compensation Award, Tranche One \| Personal Lines
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date				33.00%
Restricted Stock \| Other Key Employees \| Share-based Compensation Award, Tranche 2 \| Personal Lines
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date				33.00%
Restricted Stock \| Other Key Employees \| Share-based Compensation Award, Tranche 3 \| Personal Lines
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vested on each anniversary of the grant date				34.00%
Restricted Stock \| Non Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted		31,646	27,121	35,185
Weighted average fair value per share		$ 36.74	$ 40.42	$ 31.05
Stock Options \| Chief Executive Officer \| Share-based Compensation Award, Tranche 2 \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options awarded					300,000
Stock Options \| Chief Executive Officer \| Vesting Schedule One \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date		Dec. 31, 2018
Stock options, vested and expected to vest	100,000
Stock options forfeited	100,000
Stock Options \| Chief Executive Officer \| Vesting Schedule Two \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date		Dec. 31, 2019
Stock options, vested and expected to vest	100,000
Stock Options \| Chief Executive Officer \| Vesting Schedule Three \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date		Dec. 31, 2020
Stock options, vested and expected to vest	100,000
Stock Options \| Chief Executive Officer \| Vesting Schedule Two and Three \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, vested and expected to vest	200,000
Time Based Option Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options awarded		300,000
Stock options forfeited		0
Time Based Option Award \| Chief Executive Officer \| Share-based Compensation Award, Tranche 3 \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted		300,000
Strike price per share		$ 50
Time Based Option Award \| Chief Executive Officer \| Vesting Schedule One \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date		Dec. 31, 2018
Stock options, vested and expected to vest		100,000					100,000
Time Based Option Award \| Chief Executive Officer \| Vesting Schedule Two \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date		Dec. 31, 2019
Stock options, vested and expected to vest		100,000					100,000
Time Based Option Award \| Chief Executive Officer \| Vesting Schedule Three \| Employment Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting date		Dec. 31, 2020
Stock options, vested and expected to vest		100,000					100,000
Expiration date		Dec. 31, 2027
Scenario Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected compensation expense, options								$ 700,000	$ 1,300,000
2018 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares authorized		2,500,000					2,500,000
Accounting Standards Update 2016-09
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit on share-based compensation expense				$ 100,000
Accounting Standards Update 2016-09 \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Decreased stock based compensation expense		$ 100,000