Variable Interest Entities and Other Investments - Additional Information (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Customer	Dec. 31, 2016 Customer
Variable Interest Entities and Other Investments [Line Items]
Equity method investments	$ 10,208,000		$ 8,714,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entities and Other Investments [Line Items]
Cost method investments	11,600,000		11,200,000
Gain recognized from assets at fair value	700,000	$ 700,000
Equity method investments	10,200,000		8,700,000
Total marketable equity securities	6,100,000		$ 5,800,000
Number of loan customers | Customer				2
Number of loan customers sold | Customer			1
Net Profits interest	5,300,000		$ 5,800,000
Variable Interest Entity, Not Primary Beneficiary | Rabbi Trust
Variable Interest Entities and Other Investments [Line Items]
Defined rabbi trust assets and benefit obligation	3,800,000		3,600,000
Variable Interest Entity, Not Primary Beneficiary | Limited Partner
Variable Interest Entities and Other Investments [Line Items]
Equity method investments	9,200,000		9,200,000
Variable Interest Entity, Not Primary Beneficiary | Other Assets
Variable Interest Entities and Other Investments [Line Items]
Investments in affordable Housing Project	$ 25,200,000		$ 7,800,000