Securities - Securities Portfolio (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities		$ 0	$ 20,447
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,089,720	1,907,826
Gross Unrealized Gains		654	1,406
Gross Unrealized Losses		(49,424)	(25,023)
Fair Value		2,040,950	1,884,209
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		13,400	13,760
Fair Value		11,287
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		50,487	46,529
Gross Unrealized Gains		0	0
Gross Unrealized Losses		(296)	(184)
Fair Value		50,191	46,345
U.S. agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		160,936	157,636
Gross Unrealized Gains		146	197
Gross Unrealized Losses		(1,544)	(986)
Fair Value		159,538	156,847
Collateralized mortgage obligations (CMOs)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,213,796	1,113,019
Gross Unrealized Gains		147	121
Gross Unrealized Losses		(32,208)	(17,954)
Fair Value		1,181,735	1,095,186
Other mortgage-backed securities (MBSs)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		434,485	373,676
Gross Unrealized Gains		191	201
Gross Unrealized Losses		(11,314)	(4,334)
Fair Value		423,362	369,543
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		217,855	209,558
Gross Unrealized Gains		170	693
Gross Unrealized Losses		(4,041)	(1,260)
Fair Value		213,984	208,991
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		12,161	0
Gross Unrealized Gains		0	0
Gross Unrealized Losses		(21)	0
Fair Value		12,140	0
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[1]	0	7,408
Gross Unrealized Gains	[1]	0	194
Gross Unrealized Losses	[1]	0	(305)
Fair Value	[1]	0	7,297
Equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Securities	[1]	28,513	0
Trading securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities	[1]	0	20,447
Municipal securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		13,400	13,760
Gross Unrealized Gains		0	0
Gross Unrealized Losses		(2,113)	(1,747)
Fair Value		$ 11,287	$ 12,013

[1] (1) As a result of accounting guidance adopted in the first quarter of 2018, equity securities are no longer presented within trading securities or securities available-for-sale and are now presented within equity securities in the Consolidated Statements of Financial Condition for the current period. For further discussion of this guidance, see Note 2, "Recent Accounting Pronouncements."