BUSINESS ACQUISITIONS (Details)						3 Months Ended		12 Months Ended
	Sep. 19, 2017 USD ($) payment	Jun. 20, 2017 USD ($) shares	Jun. 14, 2017 USD ($) payment	Nov. 16, 2016 USD ($) payment	May 31, 2016 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Change in fair value of contingent consideration								$ 23,931,000	$ 0	$ 0
Goodwill								113,368,000	56,457,000	3,632,000
Acquisition-related costs								$ 1,000,000
Stock granted, net of forfeitures		$ 10,000,000
Number of reportable segments | segment								1
Current contingent consideration								$ 46,576,000	0
Contingent considerations								11,273,000	23,600,000
Contingent consideration arrangements, after tax contingent consideration expense								14,400,000
Revenue of acquirees since acquisition date								9,200,000
Earnings of acquiree since acquisition date								1,500,000
SnapCap [Member]
Business Acquisition [Line Items]
Acquisition purchase consideration	$ 11,900,000							11,886,000	0	0
Number of contingent consideration payments | payment	3
Total purchase price	$ 18,200,000							18,226,000
Contingent consideration on acquisition date	6,300,000							7,000,000
Change in fair value of contingent consideration								700,000
Goodwill	13,700,000							13,738,000
Acquisition-related costs								300,000
Earnout Payment for One Contingent Earnout [Member]
Business Acquisition [Line Items]
Contingent consideration range, maximum	3,000,000
MagnifyMoney [Member]
Business Acquisition [Line Items]
Acquisition purchase consideration		29,600,000						29,592,000
Goodwill		23,800,000						23,784,000
Acquisition-related costs								400,000
Option to acquire a foreign affiliate		$ 500,000
Option to acquire a foreign affiliate, window		3 years
DepositAccounts [Member]
Business Acquisition [Line Items]
Acquisition purchase consideration			$ 24,000,000					25,000,000	0	0
Number of contingent consideration payments | payment			7
Contingent consideration range, maximum			$ 9,000,000
Total purchase price			29,000,000					28,989,000
Contingent consideration on acquisition date			5,000,000					6,000,000
Change in fair value of contingent consideration								2,000,000
Goodwill			19,400,000					19,389,000
Acquisition-related costs								300,000
Contingent consideration, individual payments based on specified increases in federal funds interest rates			1,000,000
Contingent consideration, one-time performance payment			$ 2,000,000
Payment for contingent consideration liability, investing activities							$ 1,000,000
Current contingent consideration								1,700,000
Contingent considerations								4,300,000
CompareCards [Member]
Business Acquisition [Line Items]
Acquisition purchase consideration				$ 80,700,000				0	81,182,000	$ 0
Number of contingent consideration payments | payment				2
Contingent consideration range, maximum				$ 45,000,000.0
Total purchase price				103,800,000				103,750,000
Contingent consideration on acquisition date				23,100,000
Change in fair value of contingent consideration								21,200,000
Goodwill				52,500,000				52,450,000
Acquisition-related costs								100,000	400,000
Current contingent consideration								$ 44,300,000
SimpleTuition [Domain]
Business Acquisition [Line Items]
Acquisition purchase consideration					$ 4,500,000
Total purchase price					5,000,000
Contingent consideration on acquisition date					500,000
Goodwill					$ 375,000
Acquisition-related costs									$ 100,000
Earnout Payment for One Contingent Earnout [Member] | SnapCap [Member]
Business Acquisition [Line Items]
Contingent consideration range, minimum	$ 0
Earnout Payment for One Contingent Earnout [Member] | CompareCards [Member]
Business Acquisition [Line Items]
Contingent consideration range, maximum				$ 22,500,000.0
Two Key Employees [Member] | MagnifyMoney [Member]
Business Acquisition [Line Items]
Granted (in shares) | shares		38,468
Third Key Employee [Member] | MagnifyMoney [Member]
Business Acquisition [Line Items]
Potential future issuance of restricted stock units | shares		19,234
Subsequent Event [Member] | Earnout Payment for One Contingent Earnout [Member] | CompareCards [Member]
Business Acquisition [Line Items]
Contingent consideration range, minimum						$ 22,500,000