Investment Securities - Amortized Cost of Debt Securities Held to Maturity by External Credit Rating (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	$ 2,317,033	$ 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	138,311	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	6,794	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	289,701	297,454
Municipal bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	190,997	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	480,698	500,705
Residential mortgage-backed securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	1,616,150	1,620,119
Trust preferred securities
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	37,330	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	37,750	32,250
AAA/AA/A Rated
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	2,196,537	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	138,311	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	6,794	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	244,894	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	190,388	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	435,282	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	1,616,150	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	10,704	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	5,704	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	5,704	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-rated
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	109,792	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	39,103	43,199
Non-rated | Municipal bonds
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized cost of held to maturity debt securities	609	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	39,712	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	37,330	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	$ 32,750	$ 27,250