Investment Securities - Amortized Cost of Debt Securities Held to Maturity by External Credit Rating (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	$ 2,133,427	$ 2,336,095
U.S. Treasury securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	138,268	138,352
U.S. government agency securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	6,657	7,345
Obligations of states and state agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	280,187	297,454
Municipal bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	186,649	203,251
Obligations of states and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	466,836	500,705
Residential mortgage-backed securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	1,451,581	1,620,119
Trust preferred securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	37,335	37,324
Corporate and other debt securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	32,750	32,250
AAA/AA/A Rated
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	2,018,457	2,216,991
AAA/AA/A Rated | U.S. Treasury securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	138,268	138,352
AAA/AA/A Rated | U.S. government agency securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	6,657	7,345
AAA/AA/A Rated | Obligations of states and state agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	239,811	248,533
AAA/AA/A Rated | Municipal bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	182,140	202,642
AAA/AA/A Rated | Obligations of states and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	421,951	451,175
AAA/AA/A Rated | Residential mortgage-backed securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	1,451,581	1,620,119
AAA/AA/A Rated | Trust preferred securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
AAA/AA/A Rated | Corporate and other debt securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	5,000	10,722
BBB rated | U.S. Treasury securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | U.S. government agency securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | Obligations of states and state agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	5,722
BBB rated | Municipal bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | Obligations of states and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	5,722
BBB rated | Residential mortgage-backed securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | Trust preferred securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | Corporate and other debt securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	5,000	5,000
Non-investment grade rated
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	5,686	0
Non-investment grade rated | U.S. Treasury securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | U.S. government agency securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | Obligations of states and state agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	5,686	0
Non-investment grade rated | Municipal bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | Obligations of states and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	5,686	0
Non-investment grade rated | Residential mortgage-backed securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | Trust preferred securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | Corporate and other debt securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-rated
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	104,284	108,382
Non-rated | U.S. Treasury securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-rated | U.S. government agency securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-rated | Obligations of states and state agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	34,690	43,199
Non-rated | Municipal bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	4,509	609
Non-rated | Obligations of states and political subdivisions
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	39,199	43,808
Non-rated | Residential mortgage-backed securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-rated | Trust preferred securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	37,335	37,324
Non-rated | Corporate and other debt securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	$ 27,750	$ 27,250