Subsequent Events - Narrative (Details) $ / shares in Units, $ in Thousands		2 Months Ended	12 Months Ended
	Feb. 06, 2019 $ / shares	Feb. 27, 2019 USD ($) shares	Dec. 31, 2018 USD ($) branch $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Feb. 05, 2019 USD ($)
Subsequent Event [Line Items]
Total assets			$ 12,573,192	$ 11,915,460
Deposit			$ 10,534,513	$ 9,807,697
Common stock dividends (in dollars per share) | $ / shares			$ 0.58	$ 0.38	$ 0.30
Repurchase of common stock			$ 0	$ 0	$ 13,659
Subsequent Event
Subsequent Event [Line Items]
Common stock dividends (in dollars per share) | $ / shares	$ 0.16
Dividends, date declared	Feb. 06, 2019
Dividends, date of record	Mar. 15, 2019
Dividends, date payable	Apr. 05, 2019
Shares repurchased (in shares) | shares		176,600
Repurchase of common stock		$ 4,650
First Madison Bank & Trust
Subsequent Event [Line Items]
Total assets			258,000
Loans			202,000
Deposit			$ 213,000
Number of banking offices | branch			4
First Madison Bank & Trust | Subsequent Event
Subsequent Event [Line Items]
Merger consideration expected to be received upon closing per definitive merger agreement						$ 52,000