Premises and Equipment, Net - Premises and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Premises and Equipment, Net, by Type [Abstract]
Total cost of premises and equipment	$ 437,073		$ 425,343
Less accumulated depreciation and amortization	256,608		247,160
Premises and equipment, net	180,465		178,183
Financing lease right-of-use assets	9,326	$ 11,200	0
Assets held for sale	0		485
Total premises and equipment, net	189,791		178,668
Land [Member]
Premises and Equipment, Net, by Type [Abstract]
Total cost of premises and equipment	52,454		52,779
Building and Leasehold Improvements [Member]
Premises and Equipment, Net, by Type [Abstract]
Total cost of premises and equipment	210,362		202,912
Furniture and Equipment [Member]
Premises and Equipment, Net, by Type [Abstract]
Total cost of premises and equipment	$ 174,257		$ 169,652