Segment Information, Total Assets by Segment (Details) - USD ($) $ in Thousands	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 2,839,451	$ 2,606,217	$ 2,921,096
Operating Segments [Member] | Research [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,238,178	1,133,846	1,235,609
Operating Segments [Member] | Publishing [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	575,033	582,339	672,987
Operating Segments [Member] | Solutions [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	563,489	575,068	439,554
Corporate [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 462,751	$ 314,964	$ 572,946