Other Comprehensive Income (Loss) - Reclassification out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended													12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gains on securities transactions, net					$ 60									$ (116)	$ 1,689	$ (355)
Compensation and employee benefits expense					15,624									69,907	62,993	58,115
Total before tax					(12,661)									(33,659)	(47,080)	(49,756)
Income tax (benefit) expense	$ 3,123	$ 6,956	$ (2,961)	$ 3,805	8,983	$ 194	$ 5,934	$ 5,012	$ 4,868	$ 4,611	$ 4,052	$ 4,281	$ 3,859	10,923	16,008	16,803
Net of tax	$ (14,867)	$ (10,827)	$ 14,742	$ (11,784)	(3,678)	$ (1,514)	$ (9,272)	$ (10,293)	$ (9,993)	$ (8,497)	$ (8,233)	$ (8,313)	$ (7,910)	(22,736)	(31,072)	(32,953)
Accumulated other comprehensive (loss) income components
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax					51									2,410	14,432	8,571
Income tax (benefit) expense					(107)									(506)	(5,150)	(3,062)
Net of tax					(56)									1,904	9,282	5,509
Accumulated other comprehensive (loss) income components | Reclassification adjustment for gains included in net income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gains on securities transactions, net					60									(116)	2,626	(552)
Accumulated other comprehensive (loss) income components | Reclassification adjustment of actuarial net gain included in net income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Compensation and employee benefits expense					$ (9)									$ 2,526	$ 11,806	$ 9,123