Net Earnings Per Share (Details) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($) item $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares
Anti-dilutive securities
Number of classes of common stock | item					2
Numerator
Preferred stock dividends	$ (4,271)				$ (4,271)
Net income available to common stockholders	$ 124,034		$ 115,211		$ 262,128	$ 225,862
Denominator
Weighted-average common shares outstanding | shares	111,371,790		104,003,960		111,196,011	103,876,647
Basic earnings per share (in dollars per share) | $ / shares	$ 1.11		$ 1.11		$ 2.36	$ 2.17
Numerator
Net income	$ 128,305	$ 138,094	$ 115,211	$ 110,651	$ 266,399	$ 225,862
Preferred dividends paid					(4,271)
Assumed conversion of convertible senior notes			1,735			3,474
Net income plus assumed conversions	$ 124,034		$ 116,946		$ 262,128	$ 229,336
Denominator
Number of shares used in basic computation | shares	111,371,790		104,003,960		111,196,011	103,876,647
Weighted-average effect of dilutive securities (in shares) | shares	1,435,233		8,420,622		1,402,612	8,449,859
Number of shares used in per share computation | shares	112,807,023		112,424,582		112,598,623	112,326,506
Diluted net income per share (in dollars per share) | $ / shares	$ 1.10		$ 1.04		$ 2.33	$ 2.04
Restricted Stock Units
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted earnings per share (in shares) | shares					976,613	951,878