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Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value    
Less than 12 Months $ 622,536 $ 196,051
12 Months or Longer 93,616 103,991
Total 716,152 300,042
Gross Unrealized Losses    
Less than 12 Months 12,255 1,718
12 Months or Longer 5,023 2,796
Total 17,278 4,514
Fair Value    
Less than 12 Months 206,090 74,926
12 Months or Longer 68,297 71,079
Total 274,387 146,005
Gross Unrealized Losses    
Less than 12 Months 2,725 652
12 Months or Longer 2,669 1,431
Total 5,394 2,083
Available-for-sale Securities and Held-to-maturity Securities [Abstract]    
Less than 12 Months, Fair Value 828,626 270,977
12 Months or Longer, Fair Value 161,913 175,070
Total, Fair Value 990,539 446,047
Less than 12 Months, Gross Unrealized Losses 14,980 2,370
12 Months or Longer, Gross Unrealized Losses 7,692 4,227
Total, Gross Unrealized Losses 22,672 6,597
U.S. Government-sponsored agency securities    
Fair Value    
Less than 12 Months 371  
Total 371  
Gross Unrealized Losses    
Less than 12 Months 1  
Total 1  
State and municipal    
Fair Value    
Less than 12 Months 236,539 13,296
12 Months or Longer 28,126 35,324
Total 264,665 48,620
Gross Unrealized Losses    
Less than 12 Months 5,298 198
12 Months or Longer 1,940 893
Total 7,238 1,091
U.S. Government-sponsored mortgage-backed securities    
Fair Value    
Less than 12 Months 385,626 182,755
12 Months or Longer 65,490 68,667
Total 451,116 251,422
Gross Unrealized Losses    
Less than 12 Months 6,956 1,520
12 Months or Longer 3,083 1,903
Total 10,039 3,423
U.S. Government-sponsored agency securities    
Fair Value    
Less than 12 Months 9,861 9,988
12 Months or Longer 12,043 12,196
Total 21,904 22,184
Gross Unrealized Losses    
Less than 12 Months 258 131
12 Months or Longer 457 304
Total 715 435
State and municipal    
Fair Value    
Less than 12 Months 18,155 2,430
12 Months or Longer 15,388 15,805
Total 33,543 18,235
Gross Unrealized Losses    
Less than 12 Months 307 36
12 Months or Longer 604 208
Total 911 244
U.S. Government-sponsored mortgage-backed securities    
Fair Value    
Less than 12 Months 177,080 62,508
12 Months or Longer 40,866 43,078
Total 217,946 105,586
Gross Unrealized Losses    
Less than 12 Months 2,154 485
12 Months or Longer 1,608 919
Total 3,762 $ 1,404
Corporate obligations    
Fair Value    
Less than 12 Months 994  
Total 994  
Gross Unrealized Losses    
Less than 12 Months 6  
Total $ 6