Income taxes - Schedule of Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Employee deferred compensation plans	$ 27,423	$ 30,969
Deferred rent	10,582	10,024
Lease incentive	5,855	6,212
Broker notes	3,441	3,460
Auction rate securities reserve	2,666	3,229
Net operating loss	2,834	2,896
Involuntary conversion	2,245	2,033
Reserve for litigation and legal fees	3,350	5,808
Allowance for doubtful accounts	1,037	984
State and local net operating loss/credit carryforward	1,330	357
Other	2,718	2,650
Total deferred tax assets	63,481	68,622
Valuation allowance	126	113
Deferred tax assets after valuation allowance	63,355	68,509
Deferred tax liabilities:
Goodwill amortization	53,364	48,025
Partnership investments	7,444	10,865
Mortgage servicing rights	10,571	12,173
Company owned life insurance	6,431	6,501
Change in accounting method	1,313	2,591
Book versus tax depreciation differences	657	982
Other	248	469
Total deferred tax liabilities	80,028	81,606
Deferred tax liabilities, net	$ (16,673)	$ (13,097)