Debt Securities Available for Sale and Debt Securities Held to Maturity - Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt securities available for sale:
Amortized Cost	$ 2,497,004,000	$ 1,718,354,000
Gross Unrealized Gains	9,268,000	8,275,000
Gross Unrealized Losses	36,207,000	17,611,000
Fair Value	2,470,065,000	1,709,018,000
Net gains (losses) on debt securities	348,000	237,000	$ (581,000)
Debt securities available for sale sold during the period	251,300,000	0	0
Impairment charges on debt securities available for sale	0	0	0
Net gains on the same of debt securities available for sale	127,000
Debt securities held to maturity:
Amortized Cost	148,852,000	161,576,000
Gross Unrealized Gains	1,460,000	4,462,000
Gross Unrealized Losses	1,045,000	412,000
Fair Value	149,267,000	165,626,000
Impairment charges on debt securities held to maturity	0	0	$ (716,000)
Mortgage-backed securities: | Collateral pledged
Debt securities available for sale:
Debt securities available for sale pledged as collateral	1,600,000	900,000
Mortgage-backed securities, U.S. Government sponsored enterprises and federal agencies
Debt securities available for sale:
Amortized Cost	1,930,696,000	908,189,000
Gross Unrealized Gains	9,222,000	308,000
Gross Unrealized Losses	26,728,000	13,812,000
Fair Value	1,913,190,000	894,685,000
Debt securities held to maturity:
Amortized Cost	146,052,000	158,776,000
Gross Unrealized Gains	1,460,000	4,462,000
Gross Unrealized Losses	1,045,000	412,000
Fair Value	146,467,000	162,826,000
Mortgage-backed securities, Other
Debt securities available for sale:
Amortized Cost	4,000	6,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,000	6,000
Obligations of states and political subdivisions
Debt securities available for sale:
Amortized Cost	566,304,000	810,159,000
Gross Unrealized Gains	46,000	7,967,000
Gross Unrealized Losses	9,479,000	3,799,000
Fair Value	556,871,000	814,327,000
Other securities
Debt securities held to maturity:
Amortized Cost	2,800,000	2,800,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 2,800,000	$ 2,800,000