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%