Goodwill and Other Intangible Assets (Goodwill & Impairment Analysis) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) segment	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 465,547,000	$ 465,697,000	$ 465,547,000	$ 469,433,000
Number of reportable segments | segment	3
Goodwill impairment	$ 0