Investment Securities - Amortized Cost of Debt Securities Held to Maturity by External Credit Rating (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	$ 2,173,011	$ 2,336,095
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	68,126	138,352
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	6,222	7,345
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	262,762	297,454
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	207,497	203,251
Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	470,259	500,705
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	1,550,306	1,620,119
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	37,348	37,324
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	40,750	32,250
AAA/AA/A Rated
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	2,019,348	2,216,991
AAA/AA/A Rated | U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	68,126	138,352
AAA/AA/A Rated | U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	6,222	7,345
AAA/AA/A Rated | Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	228,286	248,533
AAA/AA/A Rated | Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	166,408	202,642
AAA/AA/A Rated | Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	394,694	451,175
AAA/AA/A Rated | Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	1,550,306	1,620,119
AAA/AA/A Rated | Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
AAA/AA/A Rated | Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	5,000	10,722
BBB rated | U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	5,722
BBB rated | Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	5,722
BBB rated | Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
BBB rated | Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	5,000	5,000
Non-investment grade rated
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	5,650	0
Non-investment grade rated | U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	5,650	0
Non-investment grade rated | Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	5,650	0
Non-investment grade rated | Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-investment grade rated | Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-rated
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	143,013	108,382
Non-rated | U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-rated | U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-rated | Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	28,826	43,199
Non-rated | Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	41,089	609
Non-rated | Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	69,915	43,808
Non-rated | Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	0	0
Non-rated | Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	37,348	37,324
Non-rated | Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Amortized cost of held to maturity debt securities	$ 35,750	$ 27,250